This adds to Zulip support for a user changing their own email
address.
It's backed by a huge amount of work by Steve Howell on making email
changes actually work from a UI perspective.
Fixes#734.
This commit loses some indexes, unique constraints etc. that were
manually added by the old migrations. I plan to add them to a new
migration in a subsequent commit.
(imported from commit 4bcbf06080a7ad94788ac368385eac34b54623ce)
Umair Khan
Vishnu Ks
Reid Barton
Luke Faraone