Bring Slack app docs to CE and add tiers

doc/user/project/integrations/gitlab_slack_application.md

-# GitLab Slack application
+# GitLab Slack application **[FREE ONLY]**
 
 NOTE: **Note:**
 The GitLab Slack application is only configurable for GitLab.com. It will **not**

doc/user/project/integrations/project_services.md

@@ -14,8 +14,7 @@ want to configure.
 
  ![Project services list](img/project_services.png)
 
-Below, you will find a list of the currently supported ones accompanied with
-comprehensive documentation.
+Below, you will find a list of the currently supported ones accompanied with comprehensive documentation.
 
 ## Services
 
@@ -48,8 +47,8 @@ Click on the service links to see further configuration instructions and details
 | Packagist | Update your project on Packagist, the main Composer repository |
 | Pipelines emails | Email the pipeline status to a list of recipients |
 | [Slack Notifications](slack.md) | Send GitLab events (e.g. issue created) to Slack as notifications |
-| [Slack slash commands](slack_slash_commands.md) | Use slash commands in Slack to control GitLab |
-| [GitLab Slack application](gitlab_slack_application.md) | Use Slack's official application
+| [Slack slash commands](slack_slash_commands.md) **[CORE ONLY]** | Use slash commands in Slack to control GitLab |
+| [GitLab Slack application](gitlab_slack_application.md) **[FREE ONLY]** | Use Slack's official application |
 | PivotalTracker | Project Management Software (Source Commits Endpoint) |
 | [Prometheus](prometheus.md) | Monitor the performance of your deployed apps |
 | Pushover | Pushover makes it easy to get real-time notifications on your Android device, iPhone, iPad, and Desktop |

doc/user/project/integrations/slack_slash_commands.md

-# Slack slash commands
+# Slack slash commands **[CORE ONLY]**
 
-> Introduced in GitLab 8.15
+> Introduced in GitLab 8.15.
 
-Slack slash commands allow you to control GitLab and view content right inside Slack, without having to leave it. This requires configurations in both Slack and GitLab.
+Slack slash commands allow you to control GitLab and view content right inside
+Slack, without having to leave it. This requires configurations in both Slack and GitLab.
 
-> Note: GitLab can also send events (e.g. issue created) to Slack as notifications. This is the separately configured [Slack Notifications Service](slack.md).
+GitLab can also send events (e.g., `issue created`) to Slack as notifications.
+This is the separately configured [Slack Notifications Service](slack.md).
+
+NOTE: **Note:**
+For GitLab.com, use the [Slack app](gitlab_slack_application.md) instead.
 
 ## Configuration