Remove Mattermost team with GitLab group

When destroying a group, now an API call is made to the Mattermost
server to request the deletion of the project.

app/services/groups/destroy_service.rb

@@ -22,6 +22,10 @@ module Groups
         DestroyService.new(group, current_user).execute
       end
+      if group.chat_team
+        Mattermost::Team.new(current_user).destroy(@group.chat_team.team_id)
+      end
+
       group.really_destroy!
     end
   end

changelogs/unreleased/zj-delete-mm-team.yml

+---
+title: Remove Mattermost team when deleting a group
+merge_request:
+author:

lib/mattermost/client.rb

@@ -36,6 +36,12 @@ module Mattermost
       end
     end
+    def session_delete(path, options = {})
+      with_session do |session|
+        delete(session, path, options)
+      end
+    end
+
     def json_response(response)
       json_response = JSON.parse(response.body)

lib/mattermost/team.rb

@@ -14,5 +14,9 @@ module Mattermost
         type: type
       }.to_json)
     end
+
+    def destroy(team_id:)
+      session_delete("/api/v4/teams/#{team}")
+    end
   end
 end