GetGravatarImageUrl: Rem. dupl. and ex

Removed duplicate GetGravatarImageUrl method in Comments.ascx.cs,
leaving the one in PresentationUtils untouched, and calling from
Comments.

Also checked for null email before calling to prevent
NullReferenceException as per bug fix request:
see https://github.com/dubeaud/bugnet/issues/53
(although I'm unclear how one goes about setting a null email unless
directly in the DB, it's always good to idiot-proof)