Handle ToS for logged in users

[annavik]

Summary

Follow up task from #513. On account creation, we want users to accept ToS from a checkbox or similar. For logged in users, we want to check if all current terms has been accepted. If not, we want to show a dialog with this info and ask for acceptance.

Sub tasks

Backend

• Add new model for Terms (field for when the user agreed, field for when the term was created or modified, field for name & short description, field for slug/key to reference in frontend)
• Add new endpoint - view all, and agree-to endpoint
• Add terms data to the current user
• Add terms check to all permission checks on the backend. Anything that requires a logged-in state needs to also check all of the terms.

Frontend

• Make it possible to accept all terms as part of account creation
• On ToS page, loop over terms, show if user has agreed and when
• On periodic checks to /me for logged-in state, also check if the user has agreed to all terms. If not, show modal with terms that need to be agreed to.