FR: make slack notifications "compatible" with mattermost
Describe the problem to be solved
We are using an on-premise mattermost server as our chat server. In general mattermost's incomming hooks are compatible with slack hooks. It is described in their documentation.
Describe the solution that you'd like
It would be great to also be able to use an on-premise mattermost server for notifications. I assume, the only addition to the current settings might be a URL to the server. Channel and token information should be more or less alike.