[BB-1069] Add Privacy Policy and Terms of Use

Created by: jamestait

• Add a checkbox to the registration form to indicate acceptance of the Privacy Policy, without which the application will not be allowed.
• Add templates for the Privacy Policy and Terms of Use, and link these from the descriptions of the relevant registration checkboxes.
• Add a field to the Instance model indicating the timestamp at which the Privacy Policy was accepted.
• Populate this field with the timestamp of the BetaTestApplication creation when creating an Instance in response to email validation, which is effectively the time at which we can be certain the Privacy Policy was accepted.

Testing Instructions

• Merge this branch to your local devstack and apply migrations.
• Navigate to the registration form and click the links for the Privacy Policy and Terms of Use.
• Attempt to submit without accepting the Privacy Policy.
  • The submission should be denied, and an error message displayed.
• Accept the privacy policy and resubmit.
  • The submission should proceed, and you will see that it is under review.
• In the DJango admin, locate the application and set its status to Approved.
• In the Django console output should be an email to validate your email address; logout of Django admin and then navigate to the link in the email.
• In the Django admin, observe the newly-created OpenedX Instance.
• Navigate to the instance and observe the Accepted Privacy Policy field.