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

4 files changed, 49 insertions, 3 deletions

diff --git a/doc/project_services/img/mattermost_configuration.png b/doc/project_services/img/mattermost_configuration.png
new file mode 100644
index 00000000000..3c5ff5ee317
--- /dev/null
+++ b/doc/project_services/img/mattermost_configuration.png
diff --git a/doc/project_services/mattermost.md b/doc/project_services/mattermost.md
new file mode 100644
index 00000000000..fbc7dfeee6d
--- /dev/null
+++ b/doc/project_services/mattermost.md
@@ -0,0 +1,45 @@
+# Mattermost Notifications Service
+
+## On Mattermost
+
+To enable Mattermost integration you must create an incoming webhook integration:
+
+1. Sign in to your Mattermost instance
+1. Visit incoming webhooks, that will be something like: https://mattermost.example/your_team_name/integrations/incoming_webhooks/add
+1. Choose a display name, description and channel, those can be overridden on GitLab
+1. Save it, copy the **Webhook URL**, we'll need this later for GitLab.
+
+There might be some cases that Incoming Webhooks are blocked by admin, ask your mattermost admin to enable
+it on https://mattermost.example/admin_console/integrations/custom.
+
+Display name override is not enabled by default, you need to ask your admin to enable it on that same section.
+
+## On GitLab
+
+After you set up Mattermost, it's time to set up GitLab.
+
+Go to your project's **Settings > Services > Mattermost Notifications** and you will see a
+checkbox with the following events that can be triggered:
+
+- Push
+- Issue
+- Merge request
+- Note
+- Tag push
+- Build
+- Wiki page
+
+Bellow each of these event checkboxes, you will have an input field to insert
+which Mattermost channel you want to send that event message, with `#town-square`
+being the default. The hash sign is optional.
+
+At the end, fill in your Mattermost details:
+
+| Field | Description |
+| ----- | ----------- |
+| **Webhook**  | The incoming webhooks which you have to setup on Mattermost, it will be something like: http://mattermost.example/hooks/5xo... |
+| **Username** | Optional username which can be on messages sent to Mattermost. Fill this in if you want to change the username of the bot. |
+| **Notify only broken builds** | If you choose to enable the **Build** event and you want to be only notified about failed builds. |
+
+
+![Mattermost configuration](img/mattermost_configuration.png)
diff --git a/doc/project_services/project_services.md b/doc/project_services/project_services.md
index a7bcd186a8c..0f398874b8f 100644
--- a/doc/project_services/project_services.md
+++ b/doc/project_services/project_services.md
@@ -44,10 +44,11 @@ further configuration instructions and details. Contributions are welcome.
 | JetBrains TeamCity CI | A continuous integration and build server |
 | [Kubernetes](kubernetes.md) | A containerized deployment service |
 | [Mattermost slash commands](mattermost_slash_commands.md) | Mattermost chat and ChatOps slash commands |
+| [Mattermost Notifications](mattermost.md) | Receive event notifications in Mattermost |
+| [Slack Notifications](slack.md) | Receive event notifications in Slack |
 | PivotalTracker | Project Management Software (Source Commits Endpoint) |
 | Pushover | Pushover makes it easy to get real-time notifications on your Android device, iPhone, iPad, and Desktop |
 | [Redmine](redmine.md) | Redmine issue tracker |
-| [Slack](slack.md) | A team communication tool for the 21st century |
 
 ## Services Templates
 
diff --git a/doc/project_services/slack.md b/doc/project_services/slack.md
index 3cfe77c9f85..0b682b43810 100644
--- a/doc/project_services/slack.md
+++ b/doc/project_services/slack.md
@@ -1,4 +1,4 @@
-# Slack Service
+# Slack Notifications Service
 
 ## On Slack
 
@@ -15,7 +15,7 @@ Slack:
 
 After you set up Slack, it's time to set up GitLab.
 
-Go to your project's **Settings > Services > Slack** and you will see a
+Go to your project's **Settings > Services > Slack Notifications** and you will see a
 checkbox with the following events that can be triggered:
 
 - Push