Restrict contact information field in arti to just allow an email address

Over the years folks have put all sorts of information into the contact information field. When working on arti relay we should be more strict than that. I think we should only allow an email address and ignore all the other stuff people try to put there. We have recently seen scammers that abuse that field for lack of any other usable one and I think with arti relay it's time to prevent that from happening in the future.

EDIT (05/30/2023): However, to be clear, this is not the only reason for this ticket. See e.g.: #870 (comment 2906733).

I have no strong opinion on whether we should allow some kind of obfuscation mechanisms like [@] instead of a plain @ but it could be something to consider.

/cc @ahf @gus