Merge branch 'feature/improve-mrwbs-and-todos-for-pipelines' into 'master'

Trigger Merge When Pipeline Succeeds on pipeline event

## What does this MR do?

This MR is meant to improve merge when build succeeds triggers, which has an impact on performance.

- [x] Move Merge When Build Succeeds trigger from commit status to pipeline event
- [x] Drop support for triggering event for branches that include commit status submitted without branch (no longer relevant)
- [x] Perform Merge When Pipeline Succeeds asynchronously to improve performance and avoid race conditions
- [x] Add missing feature test that verifies if MWBS feature actually works and merges merge requests
- [x] Update the documentation to reflect change in the behavior

Moved to separate merge request:

- [ ] Rename Merge When Build Succeeds to Merge When Pipeline Succeeds
- [ ] Update documentation to reflect name change for this feature

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing

See merge request !6675

1 files changed, 1 insertions, 0 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 2896acc6ee3..e13e0eef87a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,7 @@
 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.13.0 (unreleased)
+  - Improve Merge When Build Succeeds triggers and execute on pipeline success. (!6675)
   - Respond with 404 Not Found for non-existent tags (Linus Thiel)
   - Truncate long labels with ellipsis in labels page
   - Adding members no longer silently fails when there is extra whitespace