Use GitHub API for certain fragment checks

[katrinafyi]

I think these are the remaining cases for
client-side github fragments, and they can be emulated using the GITHUB_TOKEN and API calls.

• #readme fragment going to implicit readme file - api (thanks @mre!)
• fragments in implicit readme files (within a directory view) - api (thanks @mre!)
• issue comment numbers - use api
• pr comment numbers - use api

Originally posted by @katrinafyi in #1729. See that thread for earlier context.

Closes #1729

[katrinafyi]

I was thinking about this and I wonder if we could move away from octocrab and use the HTTP API directly. We only use a tiny portion of the features and octocrab seems to add a fair bit of build time.

My idea is we could have a quirk that replaces GitHub URLs with API URLs, then we add the auth token for URLs matching the API domain, then we check as usual. Most of our API usage is just checking status 200, and the readme endpoint just returns a rendered HTML which can go through the usual fragment checker.

It should simplify things a fair bit.