MDLSITE-2439 Add a new status field to amos_translators table

This will allow us to set explicit contributors for a language pack
(e.g. when the user stops being the maintainer).
MDLSITE-2439 Reflect the change in the amos_translators table
MDLSITE-2439 Display explicit contributors at the credits page
MDLSITE-2439 Allow turning credits page into editing mode

In editing mode, new control widgets are displayed. They allow to add a
new maintainer or contributor, or remove existing ones.
MDLSITE-2439 Handle the edit mode actions for the credits page