This is inspired by the github approach, with some adjustments,
simplifications and improvements. This approach should be supported by
all grade C browsers, except for Firefox 27-51. As such, it seems the
best solution until we can use the grid layout (T270775).
The boilerplate code for the new module was shamelessly stolen from
Ied858465c2e938828fc146880827acb6aa15fdd6.
Also noting here for posterity: this will NOT work in WMF servers (beta
or prod), since we use wikidiff2 to generate diffs there.
Bug: T285956
Change-Id: I0cb5f10254af78043b0f6250f7695c6b962fbf6b
9ef58c1d0b