FEATURE: show group mentions and topics in groups page

db/migrate/20151201035631_add_group_mentions.rb

@@ -0,0 +1,12 @@
+class AddGroupMentions < ActiveRecord::Migration
+  def change
+    create_table :group_mentions do |t|
+      t.integer :post_id
+      t.integer :group_id
+      t.timestamps
+    end
+
+    add_index :group_mentions, [:post_id, :group_id], unique: true
+    add_index :group_mentions, [:group_id, :post_id], unique: true
+  end
+end