Automatically handle old package PRs

[Octogonapus]

Assuming that:

1. JuliaRegistrator is secure and can't be tricked into posting a comment that an author created a PR who didn't actually create it

We could automatically ignore PRs if:

1. The PR introduces a package that already exists
2. We can find the PR that introduced the package already in the registry
3. Both PRs are from the same author
4. The PR being scanned is older than the one that was merged

This happens a lot because package authors sometimes:

1. Open PR 1
2. PR 1 is rejected for naming/etc.
3. Open PR 2
4. PR 2 is merged
5. PR 1 is never closed