A start on authentication improvements

This is trying to address #475 

The first commit here is mainly cleanup. The only functional difference is that the buddy fingerprint is stored on first ake and compared going forward. In the second commit I started adding some hooks for visual updates.

Some things left to do:

- [x] Store the fingerprint on ake success and compare it going forward
- [ ] Abstract the warning mechanism
- [ ] Inject an orange warning "you should verify your buddy" when conversation starts
- [ ] Inject a red warning if key changes
- [ ] Visual indication that a user is verified or not
- [ ] A manual verification button (for out-of-band auth)
- [ ] Explain what all this authentication is about

If you add your UI elements I can rebase these commits and continue. Or feel free to merge and pick things up as you see fit.