feat: Show icons for files in Merge Request changes

• Draft: Open MR Changes (!152 - closed)
• Part 1: feat: Show icons for files in Merge Request cha... (!153 - merged)
• Part 2: feat: Open MR changes (!154 - merged)

Description

This MR takes a step towards resolving Open changed files when opening MR in Web IDE (gitlab#386256 - closed) by showing an icon next to files that are in the relevant merge request changes.

In the next MR, we'll add behavior for showing the MR diffs.

Screenshots

New Icons	New Tooltip	New initializing message	New initializing message (loading MR part)

New reloading message	When loading MR fails

How to test?

1. Start local gdk and find/create a MR with a reasonable amount of changes.
2. (Optional) Setup an "Access Token" for a user
3. Start the gitlab-web-ide example app with yarn start:example
4. Visit http://localhost:8000?mrId=MR_IID where MR_ID should be the iid of the Merge Request you want to open (found in the URL of the Merge Request).
5. Enter the relevant GitLab URL and Project Path. Keep Git Ref blank.
6. (Optional) Check the I have a non-production GitLab Token and enter the "Access Token" for the user
7. Press Start GitLab Web IDE