This turns the prototype code of the Revision Slider into a MediaWiki
extension.
With this extension enabled the slider should appear on diff pages
already but it won't actually show diffs when adjusting the slider
pointers.
Things that changed so far in the prototype JS code:
- pulled out the rainbow function and made it use Math.floor instead of
the ~~ binary operator
- pulled out the API request that fetches the revisions into its own
module
- cleaned up and documented some parts of the code
The code is still very much WIP and JSCS still has a couple of
complaints.
Bug: T132576
Change-Id: I2e22365f3b93a76d5b8d3997242b5fed996c6d78
