Create relation between chat user and GitLab user and allow to authorize them [ci skip]

app/models/project_services/chat_service.rb

+# Base class for Chat services
+class ChatService < Service
+  default_value_for :category, 'chat'
+
+  has_many :chat_users
+
+  def valid_token?(token)
+    self.respond_to?(:token) && self.token.present? && ActiveSupport::SecurityUtils.variable_size_secure_compare(token, self.token)
+  end
+
+  def supported_events
+  end
+
+  def trigger(params)
+    # implement inside child
+  end
+
+  def chat_user_params(params)
+    params.permit()
+  end
+end

app/models/project_services/mattermost_chat_service.rb

+# Base class for Chat services
+class MattermostChatService < ChatService
+  def title
+    'Mattermost'
+  end
+
+  def description
+    'Self-hosted Slack-alternative'
+  end
+
+  def to_param
+    'mattermost'
+  end
+
+  def help
+    'This service allows you to use slash commands with your Mattermost installation.<br/>
+    To setup this Service you need to create a new <b>"Slash commands"</b> in your Mattermost integration panel,
+    and enter the token below.'
+  end
+
+  def fields
+    [
+      { type: 'text', name: 'token', placeholder: 'https://hooks.slack.com/services/...' }
+    ]
+  end
+
+  def trigger(params)
+    user = ChatNames::FindUserService.new(chat_names, params).execute
+    return authorize_chat_name(params) unless user
+
+    Mattermost::CommandService.new(project, user, params).execute
+  end
+
+  private
+
+  def authorize_chat_name(params)
+    url = ChatNames::RequestService.new(service, params).execute
+
+    {
+      response_type: :ephemeral,
+      message: "You are not authorized. Click this [link](#{url}) to authorize."
+    }
+  end
+end