Add support to search in multiple rooms.

[poljar]

While Riot exposes searching across a single or all rooms only. A single room might have multiple ancestors due to room upgrades. This makes it necessary to search across multiple rooms when the user wants to search in a single room.

Either Seshat should learn about room ancestors and do this transparently or it should have the ability to define multiple rooms for a search.

In both cases a better query parser would likely be necessary.