By default, skip fixing issues that already have a pull request open

[neubig]

If an issue already has a pull request open, it would be better to not try to solve it again by default. We should modify github_resolver/resolve_issues.py to fix this:

1. Change the type of "success" in the resolver result class from boolean to an enum with true, false, and skipped
2. Write code that skips all issues that have a pull request open by default
3. Add a new option that makes it possible to override this default and also create PRs when a pull request is open

Tests should be written to test all three pieces of this functionality.