Save closed/closed_by references

[martinthomson]

It would be nice to record linkages between issues, particularly the relationship between a pull request and the issue that it closes.

The graphql interface makes it a little hard to extract these, so this is non-trivial. They appear as a CrossReferencedEvent in the timelineItems field of issues and pull requests with a willCloseTarget attribute. The event list can be filtered based on the type, so it's not likely to be a huge addition to the load. Note that events should be immutable, so updating this can use skip or after fairly reliably.

Originally martinthomson/i-d-template#227

[MikeBishop]

Mostly noting for myself, we want to capture timelineItems of type CROSS_REFERENCED_EVENT since the last successful update, keeping a list of those where willCloseTarget is true and the source/target is the PR/issue in question.

Probably worth collapsing this to a list of PRs which would have closed a given issue or PR, plus a list of issues/PRs closed by each PR.