Adding email addresses

[gabrc52]

it seems like the app is telling people to invite others via email addresses. this uses the matrix.org identity server? and the invite doesn't look as good. rip rip

We should patch either Synapse or Element (or both!) so that if you try to add a MIT email address to a chat, it actually invites them through our mechanisms (via the account they already have) instead of through the identity server!