Implement a model and API for Acknowledgements

[lunkwill42]

Incidents can be Acknowledged, primarily by end users of Argus.

Semantically, an Acknowledgment is a way for a user to comment on an Incident, signalling, for example, to other users that this Incident has the attention of someone, and something is being done about it.

Functionally, in a normal list of active/open Incidents, Incidents that have an active acknowledgment should be suppressed. I.e. an API endpoint that returns a list of active Incident objects, should remove acknowledged Incidents from the list, unless specifically instructed to keep them in the result (by way of a GET parameter).

Multiple acknowledgements can be created on an Incident.

A ack model should contain:

• A relation to an Incident
• A relation to the User that made the acknowledgement
• A timestamp for when the acknowledment was made.
• A message string, entered by the acknowledging user.
• An optional expiry time.

For the purposes of filtering (as mentioned above), an Incident should be considered acknowledged when there is at least one non-expired Acknowledgement object associated with it.