Dumb down list algorithms in favour of smarter tags
This commit is a bit involved, as it factors the tag specific handling out of `/list-ordering` (and moves the `Algorithm` class one higher as a result), leaving it in the `Algorithm`. The algorithms for list ordering now only know how to handle a single tag, and this is managed by the `Algorithm` class - which is also no longer the base class for the list ordering. The list ordering algorithms now inherit from a generic `OrderingAlgorithm` base class which handles some rudimentary things.
Overall the logic hasn't changed much: the tag-specific stuff has been moved into the `Algorithm`, and the list ordering algorithms essentially just removed the iteration on tags. The `RoomListStore2` still shovels a bunch of information over to the `Algorithm`, which can lead to an awkward code flow however this commit is meant to keep the number of surfaces touched to a minimum.
The RoomListStore has also gained the ability to set per-list (tag) ordering and sorting, which is required for the new room list. The assumption that it defaults from the account-level settings is not reviewed by design, yet. This decision is deferred.
