QR-code scan to DM someone

**Is your feature request related to a problem? Please describe.**

Right now when you verify someone, you have to first ask them what their mxid is and manually DM them, and *then* start the QR verification cance.

**Describe the solution you'd like**

Shamelessly stealing from @fkwp - it'd be cool if you could scan a QR code to do both in one swoop.

**Additional context**

This was inspired by Florian asking whether we could use @hughns' QR-scan-to-login to also do QR-scan-to-verify-others in the all hands

Start by reviewing the existing QR-scan-to-login flow referenced in the issue and the verification flow in the relevant Element client. Define how one scan identifies the recipient, opens the DM, and begins QR verification; done means the combined flow works without manually entering an mxid.

Written by the indexing model from the issue text.