Allow "locked" and "advanced" checkboxes to be added to any admin setting.

This patch moves the implementation of the "advanced" and "locked" checkboxes for admin_settings to the base class - so any admin setting can use these flags. The current implementations of blah_with_advanced and blah_with_lock have been converted into wrappers which just set the flags on the base class.

This allows any setting to use any combination of locked or advanced checkboxes and makes it easier to add new ones (hidden might be nice...).

This change is 100% backward compatible and does not require any changes to existing subclasses of admin_settings.

To enable an advanced checkbox - call $setting->set_advanced(true);
To enable an locked checkbox - call $setting->set_lockable(true);

To set the defaults for the checkboxes call

$setting->set_advanced_flag_options(admin_setting::OPTION_ENABLED, true);
or
$setting->set_locked_flag_options(admin_setting::OPTION_ENABLED, false);

Note: All modules will work with this patch with 0 changes - the config values set have the same names as the previous implementations.

Discussion link:
https://moodle.org/mod/forum/discuss.php?d=220263