Merge branch 'fix-dangling-milestone-ids' into 'master'

Remove deleted milestones from merge requests

This is related to !1348 but thought it would be useful as a separate MR. This MR removes deleted milestone IDs from any merge request.

Closes https://github.com/gitlabhq/gitlabhq/issues/9643

See merge request !1350

db/migrate/20150918161719_remove_invalid_milestones_from_merge_requests.rb

+class RemoveInvalidMilestonesFromMergeRequests < ActiveRecord::Migration
+  def up
+    execute("UPDATE merge_requests SET milestone_id = NULL where milestone_id NOT IN (SELECT id FROM milestones)")
+  end
+end