Add proposed_events workflow.

[Fohlen]

This includes:

• adding a proposed_events table that inherits from events table, making the location field no longer mandatory
• adding a GRANT for aggregator to the table
• write a accept_event(url, location) that assigns a proposed event a new location and moves it to the events table
  • check if location is valid and the user has permission to use it
  • moderator role can override locations