Add migration downtime flags/notices.

db/migrate/20160721042141_add_column_user_id_to_protected_branches_access_levels.rb

@@ -4,6 +4,9 @@
 class AddColumnUserIdToProtectedBranchesAccessLevels < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
+  DOWNTIME = true
+  DOWNTIME_REASON = "This migrations adds two indexes, and so requires downtime."
+
   # When using the methods "add_concurrent_index" or "add_column_with_default"
   # you must disable the use of transactions as these methods can not run in an
   # existing transaction. When using "add_concurrent_index" make sure that this

db/migrate/20160721043623_allow_nulls_for_protected_branch_access_levels.rb

@@ -4,6 +4,8 @@
 class AllowNullsForProtectedBranchAccessLevels < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
+  DOWNTIME = false
+
   # When using the methods "add_concurrent_index" or "add_column_with_default"
   # you must disable the use of transactions as these methods can not run in an
   # existing transaction. When using "add_concurrent_index" make sure that this