WIP: Mattermost service

What does this MR do?

Creates a decent Project Service for Mattermost

Why was this MR needed?

Our webhooks don't work well together with MM, I started to checkout MM somewhere last week and found it was broken.

What are the relevant issue numbers?

Resolves #20707 (closed)

Screenshots (if relevant)

Following later

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together