Declare settings explicitly, and use generic settings form, update menu

This PR normalizes the settings management:

• declares them in settings/civimoodle.settings.php (allows settings defaults etc)
• uses the Generic Admin setting form (because why not)
• changes the admin setting URL to admin/setting/moodle (sorry - breaking change - the generic admin form expects this)
• changes how the admin setting menu item is added to the menu (less code, easier to do updates later on)

Main motivation for changing this, was that I want to add new settings, and may as well cleanup first.

The form looks roughly the same: