Allow submissions via GitHub Issues

[adam-coster]

We could make an Issue Template for submitting new links, complete with all of the fields the user can provide.

Once posted, a GitHub Action could validate the submitted data right from the Issue, and then:

• If there is an error, add a comment informing the submitter, and then re-run upon edit
• If there is no error, create a commit that adds those changes and make a PR for review

I'm not sure exactly how the latter option will work, since presumably the GH Action needs to act on behalf of a user to have sufficient permission to create a commit and PR. If it wasn't for that this would be a very straightforward thing to do. As a bonus, it also allows editing existing entries since someone could create an issue whose URL or Title is an exact match to an existing entry and then the robot could do the work.