Toggle Diff view in the Web IDE

Problem to solve

When navigating files in the Web IDE, we have an edit tab and a review tab. Currently the only way to see the diff is to switch between these two tabs on the left sidebar.

Further details

The review tab is the automatic context that the Web IDE goes in to when accessed via a Merge Request. This makes sense to provide this context, but it's not clear that you can edit files, or how to exit this view in favor of the other.

Proposal

We could eliminate the review tab and instead add a toggle to show the diff for changed files. This could either be in context per file, or for all changed files (either in the session, or compared to the target MR branch).