Add support for Wayback Machine archiving

[travisbrown]

There are some pretty fucked-up conversations happening over on the rms-support-letter issue tracker (e.g. some pretty extreme bigotry here, "We know the name of harassing person, it is… as per … . We may want to send abuses against it.", etc.).

It'd be fairly straightforward to add a command that would check for new issue threads or comments on a repo and request snapshots for all updated pages in the Wayback Machine, as a way of tracking and documenting this abuse. My cancel-culture project already has a number of tools for doing this kind of thing for Twitter accounts, and it wouldn't be hard to repurpose some of that code.

The basic approach would be the following:

1. Make a request against the Wayback CDX index to get a list of all archived pages.
2. Get the time of most recent snapshot for each.
3. Make requests against the GitHub API to identify new or updated issues, PRs, and comments.
4. Hit the Wayback save page endpoint for all pages that have been updated since the last snapshot.
5. Report all new Wayback URLs.

At some point we might also want to add Wayback search and bulk downloading tools.