Resizeable file tree in merge request

Iterating on https://gitlab.com/gitlab-org/gitlab-ce/issues/14249

Depending on screen size, and the number of files changed, the default size may be too large or too small. We should make it resizeable.

Proposal

Make the file tree resizeable.

Design

From https://gitlab.com/gitlab-org/gitlab-ce/issues/14249#note_104017127:

Design notes

• Min-width = 240px
• Max-width = 400px
• Default-width = 320px
• Hide +/- numbers if the width of the file browser is <= 260px (We'll tune this number during MR review)
• The resize handle should be 3 to 4 px wide (we can finalize the size during MR review)
• The handle is only visible on hover (#DFDFDF) and mouse down (#919191) states.
• It would be nice to store the user’s resize point in localStorage and use it in the next page load (similar to how we remember the inline vs side-by-side choice).

Links / references