Signup: better ref tracking

[fabianapsimoes]

Originally reported by @scruffian.

We use the ref URL parameter to track the source from which users reach the signup flow. For most visitors the ref parameter is undefined. We need to find all the URLs that link to signup and add ref parameters to them so that we know more detail about where our signups come from, for example:

http://wordpress.com/start?ref=homepage
http://wordpress.com/start?ref=store

[stale]

This issue has been marked as stale because it hasn't been updated in a while. It will be closed in a week. If you would like it to remain open, can you please comment below and see what you can do to get things moving with this issue? Thanks! 🙏