Show alert if guest author profile has thumbnail and is missing an email address

If a guest author profile has a thumbnail, but is missing an email address, we should show a warning / require that an email address be entered.

Otherwise, the user will experience unexpected results on the frontend.

No file or test is named in the issue. Start by locating the guest author profile form and its validation entry point; the work is done when a profile with a thumbnail cannot be saved without an email address and the user sees a warning.

Written by the indexing model from the issue text.