Backport `AutocompleteController#load_project` from EE!581.

- This is an optimization that was made in !581, and it needs to be
  backported to CE to avoid merge conflicts in the future.

app/controllers/autocomplete_controller.rb

@@ -55,11 +55,8 @@ class AutocompleteController < ApplicationController
 
   def find_users
     @users =
-      if params[:project_id].present?
-        project = Project.find(params[:project_id])
-        return render_404 unless can?(current_user, :read_project, project)
-
-        project.team.users
+      if @project
+        @project.team.users
       elsif params[:group_id].present?
         group = Group.find(params[:group_id])
         return render_404 unless can?(current_user, :read_group, group)
@@ -71,4 +68,14 @@ class AutocompleteController < ApplicationController
         User.none
       end
   end
+
+  def load_project
+    @project ||= begin
+      if params[:project_id].present?
+        project = Project.find(params[:project_id])
+        return render_404 unless can?(current_user, :read_project, project)
+        project
+      end
+    end
+  end
 end