This will then be used to know which IS to default to when unbinding the
threepid.

This changes the behaviour from using the server specified trusted
identity server to using the IS that used during the binding of the
3PID, if known.

This is the behaviour specified by MSC1915.

By default the homeserver will use the identity server used during the
binding of the 3PID to unbind the 3PID. However, we need to allow
clients to explicitly ask the homeserver to unbind via a particular
identity server, for the case where the 3PID was bound out of band from
the homeserver.

Implements MSC915.

We assume, as we did before, that users bound their threepid to one of
the trusted identity servers. So we simply fill the new table with all
threepids in `user_threepids` joined with the trusted identity servers.