Resolves #1069

The Matrix spec requires servers to provide thumbnails at (96x96, crop) and (320x240, scale) among others. [1] The avatars in Nheko's global/room profile and room settings are sized 130x130 on normal scaling and 260x260 on 2x scaling like on a HiDPI device. In both cases the avatar is requested as cropped and that way displayed at 96x96, making it look blurry.

This can be solved by requesting scaled avatars rather than cropped where appropriate, and cropping to the requested size afterwards.

HiDPI can be simulated in Qt by setting QT_SCALE_FACTOR=2.

[1] https://spec.matrix.org/v1.3/client-server-api/#thumbnails

fixes #1094

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.

Co-authored-by: DeepBlueV7.X <nicolas.werner@hotmail.de>