Improve auto string addition to produce better diffs and ease merging.

## Issue
- The diff for new strings are polluted by the need to add a comma to the previous entry.
- Having to re-add the newline at the end of file before commiting is a repetitive pain.

## Caveats to this approach
- When manually adding strings, developers need to put it above the `--end--` entry. Hopefully it is obvious without having to put verbose comments like "^--- add new string before this line ---^"
- Translators will surely ask how to translate "--end--".

static/app-strings.json

@@ -1273,5 +1273,6 @@
   "Unlink YouTube Channel": "Unlink YouTube Channel",
   "Sign In With YouTube": "Sign In With YouTube",
   "There was an error with LBRY first publishing.": "There was an error with LBRY first publishing.",
-  "Automagically upload to your youtube channel.": "Automagically upload to your youtube channel."
+  "Automagically upload to your youtube channel.": "Automagically upload to your youtube channel.",
+  "--end--": "--end--"
 }

ui/i18n.js

@@ -26,8 +26,12 @@ function saveMessage(message) {
   }
 
   if (!knownMessages[message]) {
+    const END = '--end--';
+    delete knownMessages[END];
     knownMessages[message] = message;
-    fs.writeFile(messagesFilePath, JSON.stringify(knownMessages, null, 2), 'utf-8', err => {
+    knownMessages[END] = END;
+
+    fs.writeFile(messagesFilePath, JSON.stringify(knownMessages, null, 2) + '\n', 'utf-8', err => {
       if (err) {
         throw err;
       }