[ticket/8960] Allow changing avatar path if it's not writable

This change will allow users to change the avatar path even if it's not
writable. However, this will get rid of the error message that the path is
not writable. Due to this, a minor addition to the explain message in the ACP
settings has been made.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-8960

Requires separate PR for olympus: #2313