Delete records when unsubscribing from repository

When unsubscribing from a Curry::Repository, delete all of the
Curry::PullRequests associated with that Curry::Repository.

When a Curry::PullRequest is destroyed, remove all associated
Curry::PullRequestCommitAuthor records and all associated
Curry::PullRequestUpdate records.
Curry workers return nil for invalid repositories

Specifically, the Curry::ClaValidationWorker and
Curry::ImportUnknownPullRequestCommitAuthorsWorker return nil when an
ActiveRecord::RecordNotFound error is raised. This prevents the worker chain
from continuing for pull requests that may no longer exists.