Add icon for language selection form (#6624)

Trying to fix the appearance of the login-language selector for the new elastic theme.

This is a small addition which makes the login-language plugin present an icon, like the rest of the login form fields. I have also issued a request for the login form selector to drop the custom styles attribute: https://github.com/hassansin/roundcube-login-language/issues/11. These two (small) enhancements make the login form appear perfectly when the login-language plugin is used.

Here is the corrected result with these changes applied: https://imgur.com/a/5clC67e