Improve name validation check on Mentor Account Form

[rgroves]

After successfully registering as a mentor. If a mentor goes to their account page, the form there allows the mentor to remove their first & last name and successfully save the form. This will then show a message above the form that states to enter a full name.

Removing the name entirely is something we should probably disallow.

Originally posted by @rgroves in https://github.com/WeAllCode/website/issues/820#issuecomment-763376028