Mattermost configuration frontend

What does this MR do?

Adds mattermost configuration frontend.

Are there points in the code the reviewer needs to double check?

⚠ NOTE: ⚠ We can review @zj's MR (!8070 (merged)) and this one asynchronously, we should wait for @zj's MR to complete it's backend review before we merge this branch into his. We would like @zj to do the merge.

Why was this MR needed?

Screenshots (if relevant)

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #24769 (closed)