wire up MSISDNs as 3PIDs

[matrixbot]

Created by @ matthew:matrix.org.

[ara4n]

In practice, this means:

• Make sydent able to validate MSISDNs for maintaining a MSISDN->MXID mapping for discovery purposes
• Make synapse able to track the resulting verified MSISDN 3PID as associated with an account (as we're not doing the split-out yet)
• Make riot able to register you with either MSISDN, email, or neither.
  • We don't need to do the dance that Blah did to mandate email address to protect against MSISDN reuse, as we mandate the password (or perhaps in future some other login flow) to check the user's identity.
• Make riot do an IS verification dance for MSISDNs just as it does for email addresses.
• Make riot show MSISDN 3PIDs in UserSettings sensibly
• Make riot show and manage the MSISDN associated with your account in UserSettings sensibly
• We do not get sucked into doing 2FA a la 2FA for logins element-meta#345.

[dbkr]

This should all be done now!