Store date for agreement for terms of use

In order to improve the handling of the terms of use (user has to click on a checkbox & a button to agree) we also need to store that the users agreed.

So we need some changes on the backend side:

• add new field terms_of_use_agreement_date on the user model (and so on the user table as well). Should be datetime with timezone & nullable.
• add the migration file.
• add the date to the payload of the user info endpoint.
• add another post endpoint /accept-terms-of-use (or similar) to set the agreement date (to curruent datetime)
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