Implement Web IDE button in diff tab of merge request

Problem to Solve

When users are reviewing feedback of a Merge Request they may need to make edits to those files. Currently the edit button opens a single file of the merge request limiting users from seeing the complete change and making changes across multiple files that may be part of the Merge Request.

Proposal

Inside of the Merge Request users should be directed to the Web IDE so that they can see the full context of changes and make changes across all files in the MR.

Design

	Screenshot
Edit icon is now a dropdown! 🎉
Preserves showing "Fork" callout when user can't edit on source branch.

Availability and Testing

The change will require package-and-qa job to be run. New tests including an E2E and possibly system tests will need to be added.