Instead of loading RevisionSlider only add a little button
to expand RevisionSlider on top of the diff page.
This makes RevisionSlider only steal a bit of space over the
diff, and only inserts quite a big slider to users that want
to have it visible for the particular diff.
API calls are only made once RevisionSlider has been expanded.
Bug: T141871
Change-Id: Ib312f6225b85b9ebdf4ac5d16e254a57d5cf6411
This adds tests for navigating the timeline using the back
and forward buttons.
It also makes sure that the buttons are disabled / enabled
at the correct times.
Bug: T133278
Change-Id: I9834dd3d3bf107557b15fdd105a946902d53b8f2