Draft: Add customizable default merge request title template

Marc Shawrequested to merge
customizable-mr-default-title-template into master

This MR has been split

This MR has been split into 4 smaller MRs for easier review:

  1. Database + model + feature flag: !228476 (merged)
  2. Backend logic (BuildService + MessageGenerator): !228477 (merged)
  3. REST API + Settings UI: !228478
  4. Documentation: !228479

Please review the individual MRs above. This MR will not be merged.

Original description below for reference.

Summary

Adds a project-level setting (mr_default_title_template) that allows maintainers to configure a template for the default MR title when creating new merge requests. This feature is in beta and gated behind the mr_default_title_template feature flag.

Closes #16080

Edited by Marc Shaw

Merge request reports