Make sure smart menu "locations" is filled with value

[wiebkemueller-hsh]

This is a follow-up issue to #300

As a user (admin) I would like to be notifyed by the system/form if a mandatory item is missing in order to show the smart menu I created. Since it is necessary to specify a location in order to show the smart menu in the frontend, the configuration of smart menu locations field should be a mandatory field or be filled with a default start value on creation of a new smart menu.

Notes:

• Users might be confused when all mandatory fields have been filled and the menu is still not shown.
• In case a smart menu is used as a mere field for storing data for other (future) purposes, there should be an extra item in the options list, specifying the purpose (e.g. "no location, only data storage").
• The hiding of a smart menu in preparation can be done via actions (hide).

[wiebkemueller-hsh]

This is ready for dev and time has to be found to realize it. Could be a good first issue for new developers!

[kaaermes]

field "Menu location(s)" is mandatory

Moodle 4.1.10 > OK Moodle 4.2.7 > OK