Work on event and notification system; Added more "issue_state"s; Replaced "ignored_issue" table; Removal of sessions in "delete_member" function

- "state" column of table "event" is now always filled
- splitted revocation state into 3 new "issue_state"s:
- 'canceled_revoked_before_accepted'
- 'canceled_after_revocation_during_discussion'
- 'canceled_after_revocation_during_verification'
- Added columns "notify_level" and "notify_event_id" to "member" table
- Replaced view "ignored_issue" by three new views:
- TABLE "ignored_member"
- TABLE "ignored_initiative"
- TABLE "non_voter"
- Function "delete_member" now removes "session"s
- Added member specific views on events:
- VIEW "event_seen_by_member"
- VIEW "pending_notification"