Added an index on milestones.title

Certain pages (e.g. the group wide issues page) filter miletones by
their title. Without an index this will result in a sequence scan on a
large dataset increasing the total loading time of a page.

db/migrate/20160106162223_add_index_milestones_title.rb

+class AddIndexMilestonesTitle < ActiveRecord::Migration
+  def change
+    add_index :milestones, :title
+  end
+end