[wip] Allow comment margin to appear on editable diff view

[jcansdale]

What this PR does

Previously the comment margin would only appear on pull request files that are open in a "full" editor window. This pull request allows the comment margin to also appear on editable diff windows.

How to test

1. Check Show PR comments in editor margin feature flag option
   

2. Open a pull request and checkout its branch
   

3. Right-click on a pull request file and View Changes in Solution
   

4. Toggle the check-box in the margin
   

5. Click comment glyph in margin to view comment
   

Observations

This functionality wasn't ready for prime time and has remained behind a feature flag.

What didn't work well

• Difficult to keep track of files/comments that are part of pull request
• The comment might no longer be relevant to the code it appears on
• Can't comment on code until it has been pushed

What worked well

• Extra space in full editor window to view comments

Possible future work

• Being able to mark comment threads as resolved
• Map of comments on pull request files
• Map of changed files in pull request

[jcansdale]

Comments on editable files are no longer a thing (see #2408).

Closing.