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
Fix displaying full day meetings

We were ignoring the meetings started the week before and ending the week
after at 00:00.
In fact what we want to ignore are meetings that starts the week before and
ending on the day the week starts at 00:00.

Fixes https://fedorahosted.org/fedocal/ticket/120