Note what page/domain users are on when they create an account

## Feature Request Form
### Problem you are trying to address with this feature
Publishers want to know how many users have created an account while on their content.

### Your solution
Note the page and/or domain when a user is created (if in fact the user creates an account "from" the sidebar, vs the web app.

### Add tags - did you add labels?
If this a customer requested feature, make sure to add that label.

The issue names no files, tests, or entry points. Start by locating the account-creation flows for the sidebar and web app, then clarify which page and domain data must be recorded and how completion will be verified.

Written by the indexing model from the issue text.