Support for GET /account/3pid and POST /account/3pid/delete

This adds improved support for managing email addresses (third party identifiers aka threepids aka 3pids) associated with the user's account.

In addition to the /account/auth endpoint used to enroll a new address, now we have

• GET /account/3pid to list currently-enrolled addresses
• POST /account/3pid/delete to remove an enrolled address (which requires UIA)

This is the client-side complement to swiclops!1 (merged)