Allow split editors (diff, settings, etc) to have a much smaller minimum size

Currently each side of a split editor can only be sized down to a minimum of 220px, and I would love to be able to resize them much smaller sometimes. Ideally to something less than 75px (which is about the size of the gutter).

I try to do this often in diffs when I want to focus on one side or the other for a certain period of time.

/cc @bpasero 