configure mappers explicitly

Otherwise backrefs are not created until something is queried
http://git.io/c_t_hA
Use localtime for all room-booking related times

This only affects times that are actually related to bookings/rooms.
E.g. start/end dates. Purely display datetimes such as creation time are
stored in UTC and displayed in the user's timezone.
Fix prebooking conflict bars
Holiday validation uses now date_time utils
Fixed messagebox weird padding
Pre-booking conflict check moved

* Pre-booking conflict warning appears now next to confirmation button
* Refactored overlapping methods in Reservation/ReservationOccurrences
* Added some date_time helpers to TemplateExec
* Removed old template and RH
Add missing .all()

Otherwise the preoccurrences iterator will yield zero items in the
second time it's used