Invite new users to publish their threepids to the identity server

[lampholder]

The modified registration flow will no longer cause people to publicly bind their threepids to the matrix account via the identity server even if they accept the identity server's terms and conditions on registration.

Instead, they will have to make a conscious decision to bind an email/msisdn associated with their mxid on their connected Identity Server.

We should give new users a nudge, gently advising them that if they do want their mxid to be discoverable from their threepid, they should do the needful to publish it to their chosen identity server, run by their chosen identity server provider.

Probably something to think about is whether we show this soft prompt if they haven't accepted the identity server's terms of services. My gut feeling is 'yes' as long as the whole thing is very clear and tweaked to be unintrusive.