Figure out event modification limits

[tuomas777]

There are multiple things that need to be considered related to event modifying, which currently is available only for contractors and officials, but also does not have any limits for them.

• Do we want to allow approved events to be edited? If yes, what about past ones?
• A notification to an event's contractor is sent only when the event is created, what if its location changes, perhaps to a different contract zone?
• Events cannot be created less than 7 days before, what about modifications? Do we maybe need to allow some fields to be edited still?
• If an event is linked to an inactive contract zone and its location changes, it would get linked to some different active contract zone, which is most probably not desired

Many of the issues could be tackled by not allowing location modifying if that is a feasible option.