Show @all in autocomplete list.

app/controllers/projects_controller.rb

@@ -130,7 +130,7 @@ class ProjectsController < ApplicationController
                       []
                     end
     team_members = sorted(@project.team.members)
-    participants = team_members + participating
+    participants = [{ username: "all", name: "Group Members" }] + team_members + participating
     @suggestions = {
       emojis: Emoji.names.map { |e| { name: e, path: view_context.image_url("emoji/#{e}.png") } },
       issues: @project.issues.select([:iid, :title, :description]),

lib/gitlab/markdown.rb

@@ -169,7 +169,12 @@ module Gitlab
     end
 
     def reference_user(identifier, project = @project)
-      if user = User.find_by(username: identifier)
+      if identifier == "all"
+        options = html_options.merge(
+          class: "gfm gfm-team_member #{html_options[:class]}"
+        )
+        link_to("@all", project_url(project), options)
+      elsif user = User.find_by(username: identifier)
         options = html_options.merge(
           class: "gfm gfm-team_member #{html_options[:class]}"
         )