Allow for incident alerts to go to one master channel

Currently, incident alerts are posted to separate Slack channels (one slack channel per incident)

Users would also like to have a general incident (ex: #general-incidents) channel in slack. Ideally this incident slack integration would include:

• Incident opened alert sent to #general-incidents when an incident is created

• Any timeline updates which occur during an incident to be posted within incident specific channel

• Incident resolved alert sent to #general-incidents when the incident gets resolved

• Also we don't want any alerts at all being sent to #general-incidents as these are incident specific details which should be kept separate and are spammy

Users have also requested the following to be considered:

A slack channel for incidents would provide a quick visual search for those outside of opsgenie. Other features to consider in this idea:

• A priority threshold - Ex: if users only want P1 and P2 events posted to this master channel.
• Color identification - Ex: Use red, yellow and blue borders for different priority levels
• Responders - push the incident to a master channel without a responder alert as an intermediary.