Add support for extracting info from Linear's GraphQL API

[daira]

Combined with #41, this would allow the graph to span GitHub and Linear.

See https://github.com/linear/linear/blob/master/packages/sdk/src/schema.graphql for the full schema (which is a bit daunting, but start from Issue and it's not too bad). We may want to allow getting the issues for a CustomView, so that we can configure within Linear which tickets are to be included.

Tickets are arranged in a tree within Linear, although we don't currently use parent-child relationships between tickets much, so that could be added later. For an initial version we just want to see which GitHub tickets connect to (mainly, block) a Linear ticket, so that we're not siloing information in Linear or requiring information to be duplicated between Linear and GitHub.

If we expose the graphs involving Linear tickets publically, then we would need to make sure that the titles and existence of issues in Linear don't reveal information not intended to be public (or, we could have a label in Linear that allows that info to be exposed publically). However, that can be decided on separately, since we wouldn't need to include Linear tickets in the public views initially.