Request: Batch Create Issue Mutation

[hashboard-charlie]

It looks like there's a batch update API, but there doesn't appear to be a way to batch create issues programmatically, other than hitting the create issue endpoint with 1 request per issue. This can quickly become a bottleneck both in terms of speed and in terms of hitting Linear's (understandably) rate-limit from a single host per hour.

Creating many issues is likely to be one of the top use-cases for using the Linear API rather than the UI (when the built-in importers don't suffice), so I think this is a fairly reasonable API to introduce.