Block access to rooms of disabled users or spaces

cherry-pick of 81ccea0e587 and 6f9260debb with a lot of changes.
Always require authentication to accept invitations

cherry-pick 7ebfaad923, not 100% working because the `authorization_filters`
in InvitationsController are commented and should be remade.
Add js response to spaces/show

cherry-pick 696ea275a5 with small adaptations.
Help/FAQ page as an external URL, refs #530

cherry-pick cf5fc824da
Set default timezone to UTC in the migration, forgotten at ffae80678a1
Fix error when sending webconf invitations to multiple emails, refs #724

cherry-pick 1628c88b09d6c