The delta `37/user_threepids.sql` aimed to update all the email
addresses to be lower case, however duplicate emails may exist in the
table already.
This commit adds a step where the delta moves the duplicate emails to a
new `medium` `email_old`. Only the most recently used account keeps the
binding intact. We move rather than delete so that we retain some record
of which emails were associated with which account.
Richard van der Hoff