Implement verification

[kir68k]

The client should be fully verified, preferably using a QR code.

Currently, with a room key file, the log file frequently shows:

matrix_sdk_crypto::machine: Failed to decrypt a room event: Can't find the room key to decrypt the event

This leads to missing messages in the export.

Occurs at least on my account and rooms hosted on Dendrite 0.14.1.
Not sure (yet) whether this happens on Synapse homeservers.
I do remember having issues with key files when on a new device, and trying to decrypt messages on Dendrite.

For later, see these:

• verification module
• VerificationRequest
• QrVerification