Use serialization cache in MW integration

Add prepareCacheKey() which submits HTML for serialization and saves
the resulting cache key, and tryWithPreparedCacheKey() which uses that
cache key (if available; if pending, it waits for it) for API requests.
Implemented save(), serialize() and showChanges() in terms of
tryWithPreparedCacheKey().

When opening the save dialog, run the conversion, cache it, and fire
off a prepareCacheKey(). Then use the cached conversion for save/diff/
serialize. This means we don't convert multiple times, and it causes
the prepared wikitext to be used.

Bug: 55979
Bug: 56011
Change-Id: I1d56fe88d312e9810a57d56a285ccdf4f1facf42
Merge "Use serialization cache in MW integration"