Calculate how many users decided to create an account (after "skip login" is implemented)

[neslihanturan]

Summary:

As you know, we are about to implement skip login feature #1238 as discussed here, thanks to @ujjwalagrawal17 . After this implementation is done, maybe we can count how many user browsed logged out, then decided to create a new account. It would be nice to know our contribution to number of commons users.

We need to be careful to prevent privacy violation.