Fixes #37932 - Add asterisk to email address users form

[kmalyjur]

It's a draft because I'm unsure about this solution.
Also, when submitting the form with the required email field empty, the "can't be blank" error appears - after unchecking the "mail enabled" checkbox, the error stays.

[adamruzicka]

It's a draft because I'm unsure about this solution.

I'm fine with this. If we ever rewrite the page in react then we might want do it properly, but for now I wouldn't lose any sleep over it.

Also, when submitting the form with the required email field empty, the "can't be blank" error appears - after unchecking the "mail enabled" checkbox, the error stays.

That's pretty standard with erb-based pages, validation errors don't go away until you submit the form again.