Fix user_threepids schema delta

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.