Import unknown Pull Request committers as records.

Create a Curry::ImportUnknownPullRequestCommittersWorker that
initializes a Curry::ImportUnknownPullRequest object that imports
committers that have not signed a CLA or do not have a GitHub login
associated with their commits.

A Curry::UnknownCommitter is a committer in a Pull Request with a GitHub
login who has not signed a CLA in Supermarket or does not have a GitHub
login associated with it.

Create the Curry::UnknownCommitter and Curry::PullRequest models. Create
Curry::UnknownPullRequestCommitter model to join the
Curry::UnknownCommitter and Curry::PullRequest models.

This also updates the Curry::PullRequestAnnotator to initialize with
a Curry::PullRequest instead of a Curry::PullRequestAnnotator.

Note: new classes and methods need to be documented.