Fixes for pasting images, especially under windows where the image mime type detection doesn't work as expected

- Fixes scrolling to an event not being reliable
- Adds new /goto command that can open URLs, go to events, or go to message indexes.
- Refactored ChatPage::handleMatrixUri() to contain the handling originally in Nheko::openLink(), and makes it return a boolean based on whether the URL was handled internally or not.

Sorry, no fun for you!

fixes #632

fixes #461

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.

There are probably a few things wrong with this, but I'm going to call it good enough for an initial commit