- add missing mimetype
- allow removal of images from pack
- allow GIF as a format
- don't divide size by 2 if the image is very small already

fixes #461

This fixes spaces or stickers not showing up for old databases as well
as the wrong format of the state_keys db.

see: https://github.com/vector-im/element-ios/issues/2353

As well as selecting more join rules.

Also fixes some issues where nested transactions will poison the
verification cache.

Using a dialog after username and password have been provided.