Confirm license for data upload

[opatut]

Before being allowed to upload data, a user should confirm the license that the uploaded data, or excerpts thereof, will be made public under. For this:

• Add fields to the API config for a markdown summary for what the user agrees to, a timestamp when this was last edited, and a URL to the full license text. Publish this information in the frontend config.
• Add a date field to the user table with a timestamp of when they last accepted the license terms.
• When opening the upload view, check if the user has accepted the current terms, if not, show a form to do so instead, including the summary description, the link to the full license, and a checkbox for agreement. Upon accepting, show the upload form as usual.
• Reject uploads through the API with an appropriate error code and a link to the uploads page if a user has not accepted the current terms. Make sure the OBS understands this error (i.e. create a ticket in the firmware repo).

For now, we don't need to necessarily add a real license, but we can use this feature already to notify users of the fact that

• Their data might leak because there is no absolute security and
• that the portal might accidentally publish any of their data due to software bugs in alpha and beta phase,
• the portal does publish excerpts from all tracks regardless of their "public" status,
• their data can be looked at by the site maintainers to discover problems and remove errors,
• users must only upload data they created themselves or have explicit permission to upload from the authors
• and which they would not mind being public in their entirety.

By storing the date when the user last accepted the terms, we can later change the terms and/or linked license and exclude tracks from being published that were created before the license was changed, or whose authors have not accepted the new license retroactively (OSM had this issue). Also by implementing this now, we make sure that users don't have the wrong expectations about what the uploaded data is used for or where it might end up.