[#51543113] Extract the contents of the `TermsOfService` model into a `Document` concern.
[#51543113] Add a PrivacyPolicy model.
[#51543113] DocumentsController#create can create privacy policies as well.

- Delegate to a DocumentCreator (can this be named better?) which creates the terms of service and the privacy policy
- If only one is passed in, that one gets created.
- If both are passed in, but one is invalid, none are created.
[#51543113] Upload a Privacy Policy along with the Terms of Service doc.