* Rewrote class and split into three classes: a base class, and html cache and a more generic cache to be used later.
* The new classes now use RequestContext.
* Renamed fetchPageText() -> fetchText().
* Split out new saveText() function from saveToFileCache().
* Various other cleanups and fixes.
Also fixed backwards setting of $wgDisableCounters in rebuildFileCache.php.
** Tweaked addAutopromoteOnceGroups() calls for performance
** Some doc tweaks and fixes
* Added NS_SPECIAL short-circuit on $wgTitle for file cache and removed unnecessary "is null" check
* Pushed "$action != 'raw'" check into HTMLFileCache
* Removed useless return value from performRequest()
* Added type hint to performAction()
