Delete Single Event Occurance

[leoselig]

As a user I want to delete a single occurance so that I do not have to cancel the whole series.

Acceptance Criteria:

• [ ] Deleting a single occurance is possible via a button on the occurance page (see #198)
• [ ] Delete button is only visible to the event owner (and the admin) but not, for instance, the room responsible
• [ ] Deleting a single occurance does not affect the general approval of the event
• [ ] Deleted occurances do not show up in any event lists (dashboard calender, upcomings event, ...)
• [ ] Clicking Delete opens dialog asking whether series or occurance should be affected

Tasks:

• [x] Build dialog for asking"series or occurance"?
• [x] Build that dialog in HTML
• [x] API to decline SingleOccurrence by start time
• [x] Link Delete Button on EventOccurrence page to decline SingleOccurrence (custom route)
• [x] Securitycheck in OccurrenceController#destroy
• [x] Link Delete Button to Event destruction if whole series is chosen
• [x] Controller and model (single_occurrence_event?) tests
• [x] Revoke exclusions in series when changing the event afterwards

Reminder:

• [x] If can? :show