Fix 0010687: call auth_logout before user_delete

auth_logout() does stuff that requires a valid user ID. When a user
attempts to delete their own account, we should first ensure that
they're logged out as per normal. Then we can delete their account as a
last step before redirecting them elsehwere.

The html headers/footers and redirect message have also been adjusted
for ease of use, and to ensure that the user doesn't miss the notice
about their account being deleted successfully.