Show recent searches on the search screen

The search screen's empty state now lists the queries the user searched
for and the rooms they opened from the results, newest first, capped at 25
and stored in AppSettings. Tapping a query fills the search field, tapping
a room opens it, and both are reachable with a hardware keyboard.

Rooms are resolved from their identifiers as the room list loads, and
those the user has left or is only invited to are dropped. The history is
cleared on logout.

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