Add the reminder_from field storing the FROM email address to use to send reminders

This should avoid problem of meeting having there reminders sent by the
wrong person.

Fix form
Adjust the unit-tests to account for the reminder_from field
Add the alembic revision adding the reminder_from field in the reminders table
Adjust the cron job to use the reminder_from email from the reminders table
Let's be a bit more backward compatible

This commit make the reminder_from field the default unless it is not set
in which case it will use the from_email as before
Adjust the form per @ralphbean advices
Merge pull request #117 from fedora-infra/reminder_from

Reminder from