MDL-37621 assign: Improvements to assignment default settings.

Remove locked flag from datetime settings.
Change requiresubmissionstatement to use the new admin flags instead of it's previous custom behaviour.
This means there is a language change:

AMOS BEGIN
 CPY [requiresubmissionstatementassignment_help,mod_assign],[requiresubmissionstatement_help,mod_assign]
AMOS END
MDL-37621 assign: Improvements to default settings.

Added a new adminsetting_configempty type so that module settings
which can have no sensible defaults can still be configured as advanced.