Github importer sets work item type on imported issues

What does this MR do and why?

Last remaining bit of #338003 (closed) before we can reschedule a back fill of work_item_type_id on the issues table.

Every other place where issues are created bypassing the Issue model have already been taken care of.

How to validate locally

You have to use the github importer in order to validate. Locally you can go to http://localhost:3000/import/github/new and authenticate with your github account. Import a repository that has issues in it. After import is complete, all created issues should have a work_item_type_id set.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #338003 (closed)