Feature Proposal: Allow divider in "Side-by-side" diff view to be draggable

All full-blown diff utilities (standalone applications) I've used allow the width of the diff panes to be changed by dragging the divider between said panes, to the left or right. This is useful for situations where one of the files is wider than the other, or if you're particularly interested in viewing more of one of the files, etc.

This wouldn't be such an issue if gitlab-ce#14171 was fixed.