fixes #1098

The cache is only approximate, i.e. it doesn't skip edits and similar,
but this might be good enough? Also no migration right now.

Speeds up startup by about 5x on my system.

Half the startup time is now loading the powerlevels for each room. We
can probably lazily load those too in the future.

Show either pack name, state key or (empty).

This adds functionality for viewing joined rooms and activating rooms.

fixes #136

implements MSC3700

see https://github.com/matrix-org/matrix-spec-proposals/pull/3700

fixes #1030