Review form elements missing the required indicator

This is a followup / was discovered by MDL-63424, where the missing required indicator was added to the checkbox form element.

While looking for cases, it was detected that a number of form elements don't currently support (apparently) that required indicators. Here it's the initial list:

$ grep -Lr 'element-template' *

editor_textarea.mustache

element-advcheckbox-inline.mustache

element-advcheckbox.mustache

element-checkbox-inline.mustache

element-duration.mustache

element-filetypes.mustache

element-modgrade.mustache

element-modvisible.mustache

element-passwordunmask-fill.mustache

element-questioncategory-inline.mustache

element-questioncategory.mustache

element-radio-inline.mustache

element-radio.mustache

element-selectyesno-inline.mustache

element-selectyesno.mustache

element-template-inline.mustache

filetypes-browser.mustache

filetypes-descriptions.mustache

filetypes-trigger.mustache

Note that some may be ok as they are (no sense to have the required element for them) and others could have a custom implementation (like checkbox or radio do).

But the list above needs to be reviewed and, when needed, support for the required indicator, added.

Ideally covered with tests so we don't forget about them again.

Ciao