Add horizontal scrolling for diff view

[trotzig]

For full-screen/wide snapshots, the diff view has to be horizontally scrolled in order to see everything. It would be better I think if the diff view was scrolled separately from the page so that headers and such stay in the same position.

[lencioni]

@trotzig did we do this? I seem to recall something that implemented this.

[trotzig]

Yep! Side-scrolling in combination with default zoom 1 on mobile made the experience even better.