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Do not load RevisionSlider initially, add button to load it

Description

Do not load RevisionSlider initially, add button to load it

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