Custom lexicon for issues, issue state (open, closed) and issue comments.

The case with issue_at is a bit weird since we have a circular
dependency: the issue record requires the issue_id, and the issue entry
in the db requires the issue_at.

To resolve this we write to the db without the issue_at, fetch the
issue_id, create the issue record on the PDS, and then update the
issue_at (with SetIssueAt). It's not great, but whatever.

also fix nix builds