user/about.json: Do not include private values in json.

> 14:26 < bboe> you can see if anyone has the over_18 attribute 

Moves some private values (which we were already returning as "None" otherwise) into a system to just not include them at all.

Also fixes the over_18 attr being visible to everyone.

This specific request removes has_mail, has_mod_mail, modhash, and over_18 for users except for the case of a user who is logged in looking at their own profile.

Ideally these would be moved into me.json, however, this would risk breaking existing api clients.