Show merge request discussion icon in Web IDE Gutter

Problem to Solve

When working on a Merge Request in the Web IDE there is no indication of which lines have comments or feedback. This makes it hard to determine which file/lines need to be addressed for feedback.

Proposal

In the gutter of the Web IDE we should show a visual indication that there is a comment on that line of the file. This icon should link to the comment in the MR so that you can see what the feedback is.

Design Proposal

Add a comment icon to lines containing comments.

• Icon should link to the associated comment
• Clicking the icon should open up the merge request comment link

What's not in Scope

• Details of what the comment says or feedback
• Whether or not the comment is resolved or still open

Availability and Testing

Please assign a SET to the MR for review. New E2E test is required - gitlab-org/quality/testcases#1100 (closed).