[IMP] Allow landscape page orientation for badges

Useful e.g. when creating certificates (one page per attendant)
[FIX] Do not clone alarms with absolute date
[FIX] UN-link coordinator when he's removed
[FIX] icon calendar misplaced in firefox
[MIN] Remove annoying/useless debug msg
[IMP] Better scheduler start/stop behavior

- track hostname and pid
- sync process status with db status
[IMP] Show scheduler hostname/pid in admin page
[IMP] Prefer event tz over system tz for guests
[FIX] scheduler tests
[IMP] Memoize EndpointURL url_for

provides a speedup in case lots of urls are build, e.g. in large
timetables (about 50% faster)
[FIX] Removed repeated bookings from calendar
[FIX] "Join Vidyo" tooltip formatting (+jquerify)
[REF] Semicolons... we do not want them!
[IMP] Sanitize HTML for PDF using bleach
[FIX] UTF8-encode HTML returned by markdown filter
[FIX] Error when memoizing URLs with list args
[IMP] Allow social events to be mandatory
[FIX] Missing mandatory check
[FIX] Allow added participants to answer invitation
[FIX] UTF8 error during abstract file uploads