Responsively switch IDE diff style
Depending on screen size we should switch the IDE diff style so that it is usable on small screens where side by side diffs don't work.
In the future we should allow users to override this and save their preference between all editing sessions https://gitlab.com/gitlab-org/gitlab-ce/issues/44304
Proposal
If the central area of the IDE doesn't allow at least 120 characters side by side, automatically switch to inline diff.
If the commit panel and file tree panel are resized, the diff style should be updated accordingly.
Edited by James Ramsay (ex-GitLab)