Reject event functionality

[ariaru]

...for events that will not be approved:

• add 'rejected' attribute to Event (as Boolean)
• add 'Reject' button to admin page for each unapproved event (via which that attribute can be set to true)
• group rejected events together in a separate list on admin page
• when event is rejected: send mail to event organizer saying that it was rejected (or should this be done manually by the admin, giving reasons, etc.?)

What do you think about this @anikalindtner ?
(It came up during our meeting last night, because the list of unapproved events is getting longer and we were wondering whether you could use something like this.)