Add a userId based on sessions

[fkh]

Originally suggested by Georgia Bullen --

It'd be helpful to have a way to track that places came from one user (even if they didn't enter a user name or if you are doing anonymous data collection), so something like a session id or a userId would be helpful!

[BenSturmfels]

Suggest closing - this feature now exists where each submitted place/comment has a user_token based on the session:

We've used this to identify submissions made by the same visitor, such as for removing abuse submissions.