Use account_threepid_delegate for sending SMS

[dbkr]

Do equivalent to what we've done for email, ie. use the account_threepid_delegate to send sms and don't require the client to send an id_server param to /msisdn/requestToken. Hopefully the m.require_identity_server flag can be re-used to signal to the client that an id_server is not required for msisdn token request (apart from for binding, as with email).

Unsure how much of this is already done.

[anoadragon453]

I think this issue is a bit out of date and has some misinformation in it.

The current state is that we have an account_threepid_delegates dict with email and msisdn keys. If msisdn is set to a domain name, msisdn-related requests are sent via that domain. If it's empty, all msisdn- related tasks return an error stating that the homeserver does not support this.

[anoadragon453]

Closing this for now.