Ability for integrations to interact with Incidents and fetch details about them

[nilmerg]

Integrations may want to integrate certain features of Icinga Notifications Web in their own UI. Such as:

• Manage/Unmanage
• Subscribe/Unsubscribe
• Who has been notified?
• Who is manager or subscriber?
• Is the incident muted?
• Is there an incident?

Since sources identify their objects based on tags, it's probably best to provide a way to access incidents based on them.

A quick idea floating around might be an Incidents object that expects an array of tags and yields objects that allow the operations mentioned above.