Allow Channel Override for Slack

[grokdesigns]

Feature Request

What problem are you looking to solve?

Allow user to enter which channel the Slack integration should post to. This is a matter of adding a field to the Connections configuration for Slack on the front end and adding a "channel": "#other-channel" override to the JSON payload on the backend.

Other Information

Currently a custom webhook must be configured solely for Radarr so that alerts get pushed to the correct channel. Other apps allow you to specify which channel they get posted to, so the webhook can be used for multiple services, which then post to their respective channels.

Also requested in Sonarr.

[grokdesigns]

Confirmed working in latest build 0.2.0.903.