Editor campaigns/Cookie notes

Current
A cookie is set when you visit AccountCreation using a campaigns URL. The original reason for doing this was so if you left AccountCreation and came back, the account creation would still be logged.

Proposal
Instead of setting a cookie, add a hidden field to the create account form. This is already the way the new opt-out works.

This will be transparent: users will be able to see the campaigns URL parameter that triggers the message and opt-out controls. Goes with a message like, "You followed a URL to join the campaign...".