Add experimental support for persistent authentication and key storage

[superp00t]

This change adds opt-in support (via a checkbox) for storing keys locally, as well as a list of authenticated users and their associated fingerprints.

[ayyghost]

This seems to break when you have more than one client open in the same browser. Steps to reproduce:

1. Open two clients in separate tabs (A and B).
2. Enable persistence for both of them.
3. On A, click "Display Info" under B's name.
4. B will no longer see any chat messages.

[ayyghost]

I really only want the commits pertaining to persistent authentication at this time. Probably going to cherry pick those manually.