Markdown: don't convert '[label](uri)' to embeds.

## Issue
Closes 4936: Don't process markdown formatting as lbry:// url previews

## Approach
Preamble:
 - We want to convert plain `https://lbry.tv/befreeonlbry` to an embed, but not `[blah](https://lbry.tv/befreeonlbry)`.
 - At the markdown/remark level, both formats resolve to the same node type, having a `link` and a `text`, with the 'text' being auto-filled with the `href` if there is no label.

Fix by assuming the link is the non-labelled format if the `text` is the same as `href`.

This opens up one corner-case that we can't handle, which is when the user explicitly set the label using the href, e.g. `[https://lbry.tv/befreeonlbry](https://lbry.tv/befreeonlbry)`. This will still resolve to an embed. There's not enough data at the parsed level differentiate this case -- we would need to parse the content ourself before `remark`, which I think is not worth it.

## Aside/Reminder
If you see that the link doesn't resolve to an embed regardless of the format used, that's probably just due to `5636: Disable video previews in comments/posts made by channels below a certain channel staked level`

ui/component/markdownLink/view.jsx

@@ -67,7 +67,10 @@ function MarkdownLink(props: Props) {
       const possibleLbryUrl = linkPathPlusHash ? `lbry://${linkPathPlusHash.replace(/:/g, '#')}` : undefined;
 
       const lbryLinkIsValid = possibleLbryUrl && isURIValid(possibleLbryUrl);
-      if (lbryLinkIsValid) {
+      const isMarkdownLinkWithLabel =
+        children && Array.isArray(children) && React.Children.count(children) === 1 && children.toString() !== href;
+
+      if (lbryLinkIsValid && !isMarkdownLinkWithLabel) {
         lbryUrlFromLink = possibleLbryUrl;
       }
     }