Increase incident_key to 200 chars

With descriptive hostnames in Icinga we regularly get incident key lengths > 100 characters, so creating new events via the api results in the error below. CharFields become varchars in sqlite and MySQL at least and they're cheap so set it much higher

'Error code: 400, Reason: {"errors": ["Ensure this value has at most 80 characters (it has 107)."]}'