WebSockets via Rocket's Upgrade connection

This PR implements a (not yet fully released) new feature of Rocket which allows WebSockets/Upgrade connections.
No more need for multiple ports to be opened for Vaultwarden.
No explicit need for a reverse proxy to get WebSockets to work (Although I still suggest to use a reverse proxy).

- Using a git revision for Rocket, since `rocket_ws` is not yet released.
- Updated other crates as well.
- Added a connection guard to clear the WS connection from the Users list.

Fixes #685
Fixes #2917
Fixes #1424