Previously we used the topic "Realm.domain" for new user signups, but topic
"Realm.string_id" for the realm creation. This changes the user signup
messages to be on the same topic thread as the realm creation.
This makes life a lot easier for people inviting users to a new Zulip
organization, since they can give some form of context now.
Modified by tabbott to clean up CSS, backend code flow, and improve
the formatting of the emails.
Fixes: #1409.
A lot of care has been taken to ensure we're using the realm that the
message is being sent into, not the realm of the sender, to correctly
handle the logic for cross-realm bot users such as the notifications
bot.