Add `allow_maintainer_to_push` to `merge_requests`

This flag will allow maintainers of a project to push to specific
branches of a fork of a project

db/migrate/20180221151752_add_allow_maintainer_to_push_to_merge_requests.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddAllowMaintainerToPushToMergeRequests < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_column :merge_requests, :allow_maintainer_to_push, :boolean
+  end
+
+  def down
+    remove_column :merge_requests, :allow_maintainer_to_push
+  end
+end

db/schema.rb

@@ -1145,6 +1145,7 @@ ActiveRecord::Schema.define(version: 20180307012445) do
     t.boolean "discussion_locked"
     t.integer "latest_merge_request_diff_id"
     t.string "rebase_commit_sha"
+    t.boolean "allow_maintainer_to_push"
   end
 
   add_index "merge_requests", ["assignee_id"], name: "index_merge_requests_on_assignee_id", using: :btree