Implement an API endpoint for manually resolving an Incident

[lunkwill42]

An Argus end user should be able to resolve an Incident manually (this was discussed in #53). Several things should happen when this operation is selected by a user:

• The operation must be logged to the Event table (the API should support adding a message for the event log, so the user can specify why they resolved an incident).
• The Incident's end_time attribute must be set to the current time, to indicate is has been closed.
• A notification containing the details of the event (who did what, when, and why/message) must be added to the notification queue.

Argus must expect that some message about an already resolved Incident might come in from a source system. This should not change the Incident record, but should be logged to the Event table.

[lunkwill42]

So, this seems to now be possible by posting a CLOSE type event to an incident.