Do not precheck newsletter field if user is marked as being in GDPR

[mwvolo]

Describe the bug
When an instructor signs up on accounts.openstax.org, the newsletter option is pre-selected.
We should not have this option preselected when a user is signing up from the EU. We should be able to filter out users using the is_gdpr flag that already exists in accounts.

Expected behavior
Newsletter is preselected for users outside the GDPR