Implements email confirmation

Support for confirming initial email address, as well as changing a
confirmed email address (confirmed email address will remain on record
until provisional address is confirmed).

Adds emailConfirmed field to User model, and corresponding database
migration.

Small changes to the email module.

Updates requirements.txt and config.sample.py.
Need style arguments here and on the expiration flash, ie ``'success'```and ```'danger'```