Better alembic filenames, simplified migration tpl

We don't really need a data migration function by default. In most cases
data migrations would be closely related to schema migrations anyway and
thus splitting them in different functions would be overkill. And if
not, we can always create functions as necessary.
Rename columns

Blocking.created_by => created_by_id
Blocking.created_at => created_dt

Photo.small_content => thumbnail
Photo.large_content => data

ReservationOccurrence.start => start_dt
ReservationOccurrence.end => end_dt
ReservationOccurrence.is_sent => notification_sent

Reservation.created_at => created_dt
Reservation.created_by => created_by_id
Reservation.uses_video_conference => uses_vc
Reservation.needs_video_conference_setup => needs_vc_assistance
Reservation.needs_general_assistance => needs_assistance
Reservation.start_date => start_dt
Reservation.end_date => end_dt

NonBookablePeriod.start_date => start_dt
NonBookablePeriod.end_date => end_dt