Render GFM on snippet comments

What does this MR do and why?

Fixes Snippet comments don't appear to render front-e... (#582767 - closed) by correctly rendering GFM on "deprecated notes" loads (currently just snippet comments, I think), and ordering the existing renderGFM calls on note add/update correctly so they actually work.

The whole of deprecated_notes_spec.js has been quarantined for 5 years, which made testing this change hard. Many of the existing specs don't pass at all locally, so I've opted to drop them all into a new quarantined describe and unquarantine the top-level one (since its beforeEach and afterEach are useful), making it possible to add new specs again.

Before	After

How to set up and validate locally

1. Go make a snippet comment with a Mermaid diagram in it:

   ```mermaid
   graph TD;
       A-->B;
       A-->C;
       B-->D;
       C-->D;
   ```

2. Notice it's not rendered as it's added.

3. Refresh the page, and notice it's still not rendered.

4. Check out the branch.

5. Load the page. Notice it's rendered on load now!

6. Add a new comment with the diagram. Notice it's loaded as it's added, too.

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