Add small HtmlCacheUpdater service class to normalize purging code

Add small HtmlCacheUpdater service class to normalize purging code

The purge() method handles purging of both file cache and CDN, using
a PRESEND deferred update. This avoids code duplication and missing
file cache purge calls.

Also:
* Migrate HTMLCacheUpdate callers to just directly using HTMLCacheUpdateJob
* Add HtmlFileCacheUpdate class and defer such updates just like with CDN
* Simplify HTMLCacheUpdate constructor parameters
* Remove BacklinkCache::clear() calls which do nothing since the backlink
  query does not actually happen until the job runs

Change-Id: Ic453b189a40109a73a9426538608eea87a76befa