Is there a pull request template that does this?
Edit: I was worried about possibly needing to change license. For now I will just use a permissive license. The situation is made seemingly complicated by the possible need to use copylefted images, combined with the possible need for using server code (which shouldn’t use creative commons) in addition to the static html. I would rather deal with including parts with different licenses (probably not as complicated as I initially thought) instead of contributor license agreements.
Edit 2: Also, license enforcement is not very important for my project.
Edit 3: Now I’m using creative commons zero and making the repo comply with https://reuse.software/
It would mean that the owner could take that code and make it closed source. They could do literally anything they wanted with it, because they would own the copyright.
They can’t make it closed source retroactively (well, technically you can design a license like that but that’s a different discussion and the most widely used open source licenses aren’t made like that). They can relicense at some point going forward, but all the code up to that point would still be available under the old license and contributors could fork and continue without batting an eye.