Allow Slack notification API URL override
Plus Target Version:
Slack notifications were added in #12291 .
Currently it's hardcoded to always post to `https://slack.com/api/chat.postMessage`.
This is fine when actually using Slack, however there is other (open source) chat software that mimics the Slack API.
Most notably RocketChat does this and can be self hosted.
Therefore please consider adding an API-URL field to the configuration in order to allow also using third party APIs.
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