Work on new features; Slight change of behaviour regarding revocation of initiatives

New features:
- possiblity for members to store issue and voting comments
- initial work on notification system (new columns in table "membership" and new "event" table)
- added new type "issue_state" and new column "issue"."state"

Other changes:
- revocation of last initiative closes issue without delay, if issue has not been accepted yet
- moved table "contingent" up (after "system_setting" table)