Allow groups to be used as aliases for user mention

when configured by the admin a group can be found through the @mentions
feature in both the compose/reply and the private message user-selectors
and once selected the mention will be replaced by the list of users in
the group

db/migrate/20140102104229_add_alias_level_to_groups.rb

@@ -0,0 +1,5 @@
+class AddAliasLevelToGroups < ActiveRecord::Migration
+  def change
+    add_column :groups, :alias_level, :integer, default: 0
+  end
+end