Configure Mattermost/Slack chat commands at instance level

Description

For single-tenant installations of GitLab, including the I2P demo, teams should be able to enable a single chat command integration (Mattermost or Slack) for the entire instance. This implies that the command structure for the chat commands would allow specifying specific projects.

Proposal

Links / references

  • Depends on

Documentation blurb

(Write the start of the documentation of this feature here, include:

  1. Why should someone use it; what's the underlying problem.
  2. What is the solution.
  3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)

/cc @victorwu @mydigitalself @bikebilly