Improved Group Member Management on User Administration

Allows for a quick and easy group membership management on the
user-administration page. Uses the select2 UI component to
autosuggest other groups, remove existing ones and lock in automatic
groups.

config/routes.rb

@@ -70,6 +70,8 @@ Discourse::Application.routes.draw do
       put "unblock"
       put "trust_level"
       put "primary_group"
+      post "groups" => "users#add_group", constraints: AdminConstraint.new
+      delete "groups/:group_id" => "users#remove_group", constraints: AdminConstraint.new
       get "badges"
       get "leader_requirements"
     end