Support Slack notifications to Mattermost webhook URLs

#1

My company uses a self-hosted Slack clone called Mattermost. It is generally compatible with Slack webhooks.

The Mattermost webhook URL has the format:
https://subdomain.domain.com/hooks/xxxxxxxxxxxxxxxxxxxxxxxxxx

Attempting to set up a Slack notification with this webhook URL fails with the message “Disabled Automatically by Buildkite: Invalid Slack URL”.

Please consider allowing Slack style notifications to Slack compatible (but non-Slack) webhook URLs. Thank you.