[REF] Notification template base class

- Closes #890
[FIX] Notification template unnecessary args
[FIX] Participant status mismatch

* Participant status set to 'present' after start of the meeting by default
* Participant are not marked as 'present' until they accept invitation
* Show only confirmed participants on the event page
* Removed unused methods
* closes #1175
[IMP] Slight wording and workflow changes

 * Allow "Accepted/absent" -> "excused";
 * see #1175;