Allow to quickly see what matters in a diff

[digulla]

In the diff view, I see the whole text and I have to scroll through all of it to find where something was changed.

Possible improvements:

• Support a compressed view which just shows the changes
• as above but with N lines of context
• Compressed view but show the current article as context (i.e. everything between two level 1 headings)
• Show whole text and add navigation buttons to move between diffs