Verification suport

[poljar]

This PR adds support for interactive verification, this needs a couple of things for everything to work correctly:

• React to verification requests over to-device messaging
• React to verification requests in DMs
• Command to request self verifications (this is over to-device messaging)
• Command to request verification of other users
• Command to accept/cancel/confirm active verifications
• Support rendering of emoji verifications
• Support rendering of QR code verifications

The command to control an active verification is called /verification it has the accept, confirm, cancel, and use-emoji subcommands.

The plan for the command that enables verifications to be requested should be called /verify:

/verify <user_id> [<device_id>] [-fingerprint <device fingerprint>].

This would allow us to request verification with a user, or with a specific device if the device id is given, alternatively if we provide a fingerprint using the -fingerprint switch a user or device is manually marked as verified.

This closes: #32.