Refactor RCFeed configuration (backwards compatible)

Refactor RCFeed configuration (backwards compatible)

Previously:
* Engines had to be registered in $wgRCEngines.
* The RCFeedEngine classes took no constructor arguments and
  were expected to send whatever text is previously formatted
  without any information about it. This generic design was
  flexible in allowing one to use any formatter with any engine
  with minimal configuration and no need for additional classes.
* Each feed configured their destination by setting a 'uri'
  option that encodes the name of the engine in PHP as the uri
  scheme. Other uri components had to be used for any other
  parameters to the engine (host, port, path). While fairly
  limited, it was sufficient for the default engines in core.

Changes:
* Allow feed classes to be directly associated with a feed in $wgRCFeeds
  via a new 'class' option - without the indirection of 'uri' and
  $wgRCEngines. All options are passed to the given class constructor.
  This matches the design used elsewhere in MediaWiki. (ObjectCache,
  FileRepo, FileBackend, JobQueue, LBFactory, etc.)

  This means we no longer enforce a 1:1 mapping of internet protocols
  to a specific feed engine, and it allows settings to be passed
  without being encoded as a URI neccecarily.

  Main use case for this refactor is EventBus (see I7edc4d57fa),

  Interestingly, this matches the (then incorrect) documentation
  written for $wgRCFeeds in 2961884b43 (which mentions an 'engine'
  property that would do the same thing).

* Move the default 'omit' filters and unrestricted 'formatter' handling
  to a new FormattedRCFeed class, which remains the default.

* Deprecate RecentChange::getEngine() in favour of RCFeed::factory().

* Document wgRCEngines as "@since 1.22". Follows 2961884b43, ffc71cb6af.

Change-Id: I8be497c623c5d928762e3d3406a388f4d91add9a