Merge branch 'issue_22269' into 'master'

Mattermost Notifications Service

## What does this MR do?
closes #22269 

## Screenshots 

![mattermost](/uploads/de71c121f544a91305b6dfa6dc4c5738/mattermost.png)  
![slack](/uploads/081d75d49239319d94332abda214fb98/slack.png)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)


See merge request !7764

1 files changed, 9 insertions, 1 deletions

diff --git a/lib/api/services.rb b/lib/api/services.rb
index b1e072b4f47..59232c84c24 100644
--- a/lib/api/services.rb
+++ b/lib/api/services.rb
@@ -473,7 +473,7 @@ module API
           desc: 'The description of the tracker'
         }
       ],
-      'slack' => [
+      'slack-notification' => [
         {
           required: true,
           name: :webhook,
@@ -493,6 +493,14 @@ module API
           desc: 'The channel name'
         }
       ],
+      'mattermost-notification' => [
+        {
+          required: true,
+          name: :webhook,
+          type: String,
+          desc: 'The Mattermost webhook. e.g. http://mattermost_host/hooks/...'
+        }
+      ],
       'teamcity' => [
         {
           required: true,