ApiComparePages: Update for MCR

ApiComparePages: Update for MCR

The main external change here is that it can now return diffs per slot,
and the various parameters for providing text are deprecated in favor of
templated per-slot versions.

Also, this deprecates the 'fromsection' and 'tosection' behavior
introduced for T183823 (extracting a section's content for the diff) in
favor of the more logical behavior requested in T185723 (expanding
'fromtext-{slot}'/'totext-{slot}' as if for a section edit).

Bug: T200569
Bug: T183823
Bug: T185723
Change-Id: I700edfa766bbc320887f2e0b7507fcdb11e72cdc