Support for multiple string context + "About" as initial example.

## Issue
4796 - i18n: Allow support for string overloading (multiple contexts)

## Approach
- Minimal code and process change.
- Handle on a case-by-case basis when reported by translators.
- Split the affected key in the string json by appending the context.
- Translators need to be aware of the new format and not translate context itself. Code is added to detect bad translations and will revert to English.

Sample in json:
  "About --[About section in Help Page]--": "About",
  "About --[tab title in Channel Page]--":  "About",

Sample in client code:
  title={__('About --[About section in Help Page]--')}

- "--[ ]--" was chosen as it's unique enough (unlikely for real strings to use it) and hopefully not that distracting in the client code.
- In the key itself, spaces are allowed after the string (i.e. before '--[') for neatness. It will be trimmed by the system.

## First example
"About" is used in 3 places:
   - Channel Page
   - Help Page
   - Footer (in Odysee branch)

For Russian, the word "About" is "O" and is usually not used standalone, but requires something behind it. A translator said so, and seems to be the case in other sites as well.
   "O xxx"
   "O yyy"

## Other languages
For other languages that are not impacted, they can just clone the same translation for each of the split keys, just like in English.

## Possible enhancement in Transifex
I see that Transifex's API includes a `context` entry. It might be possible to move the context-metadata there during the upload, so translators will never see the "--[]--" messiness (it will be shown as "Context: xxx" in the Transifex GUI).

I'm not sure how to test the Transifex side, so I did not investigate further.

static/app-strings.json

@@ -88,7 +88,8 @@
   "Find New Channels": "Find New Channels",
   "Discover New Channels": "Discover New Channels",
   "publishes": "publishes",
-  "About": "About",
+  "About --[About section in Help Page]--": "About",
+  "About --[tab title in Channel Page]--": "About",
   "Share Channel": "Share Channel",
   "Go to page:": "Go to page:",
   "Enter a URL for your thumbnail.": "Enter a URL for your thumbnail.",

ui/i18n.js

@@ -21,14 +21,14 @@ function saveMessage(message) {
     try {
       knownMessages = JSON.parse(fs.readFileSync(messagesFilePath, 'utf-8'));
     } catch (err) {
-      throw 'Error parsing i18n messages file: ' + messagesFilePath + ' err: ' + err;
+      throw new Error('Error parsing i18n messages file: ' + messagesFilePath + ' err: ' + err);
     }
   }
 
   if (!knownMessages[message]) {
     const END = '--end--';
     delete knownMessages[END];
-    knownMessages[message] = message;
+    knownMessages[message] = removeContextMetadata(message);
     knownMessages[END] = END;
 
     fs.writeFile(messagesFilePath, JSON.stringify(knownMessages, null, 2) + '\n', 'utf-8', err => {
@@ -53,6 +53,31 @@ function saveMessage(message) {
 }
 // @endif
 
+function removeContextMetadata(message) {
+  // Example string entries with context-metadata:
+  //   "About --[About section in Help Page]--": "About",
+  //   "About --[tab title in Channel Page]--": "About",
+  const CONTEXT_BEGIN = '--[';
+  const CONTEXT_FINAL = ']--';
+
+  // If the resolved string still contains the context-metadata, then it's one of the following:
+  // 1. In development mode, where 'en.json' in the server hasn't been updated with the string yet.
+  // 2. Translator made a mistake of not ignoring the context string.
+  // In either case, we'll revert to the English version.
+
+  const begin = message.lastIndexOf(CONTEXT_BEGIN);
+  if (begin > 0 && message.endsWith(CONTEXT_FINAL)) {
+    // Strip away context:
+    message = message.substring(0, begin);
+    // No trailing spaces should be allowed in the string database anyway, because that is hard to translate
+    // (can't see in Transifex; might not make sense in other languages; etc.).
+    // With that, we can add a space before the context-metadata to make it neat, and trim both cases here:
+    message = message.trimEnd();
+  }
+
+  return message;
+}
+
 export function __(message, tokens) {
   const language = localStorageAvailable
     ? window.localStorage.getItem('language') || 'en'
@@ -62,9 +87,8 @@ export function __(message, tokens) {
     saveMessage(message);
   }
 
-  const translatedMessage = window.i18n_messages[language]
-    ? window.i18n_messages[language][message] || message
-    : message;
+  let translatedMessage = window.i18n_messages[language] ? window.i18n_messages[language][message] || message : message;
+  translatedMessage = removeContextMetadata(translatedMessage);
 
   if (!tokens) {
     return translatedMessage;

ui/page/channel/view.jsx

@@ -200,7 +200,7 @@ function ChannelPage(props: Props) {
       <Tabs onChange={onTabChange} index={tabIndex}>
         <TabList className="tabs__list--channel-page">
           <Tab disabled={editing}>{__('Content')}</Tab>
-          <Tab>{editing ? __('Editing Your Channel') : __('About')}</Tab>
+          <Tab>{editing ? __('Editing Your Channel') : __('About --[tab title in Channel Page]--')}</Tab>
           <Tab disabled={editing}>{__('Discussion')}</Tab>
         </TabList>
         <TabPanels>

ui/page/help/view.jsx

@@ -212,7 +212,7 @@ class HelpPage extends React.PureComponent<Props, State> {
         {/* @endif */}
 
         <Card
-          title={__('About')}
+          title={__('About --[About section in Help Page]--')}
           subtitle={
             this.state.upgradeAvailable !== null && this.state.upgradeAvailable ? (
               <span>