Add maintainer github issue template

[QMalcolm]

Resolves #178

Description

This is a copied and modified version of the dbt-core implementation issue template. This is nice to have because the existing issue templates are good templates for community members but feel odd for internal issue creation. This gives us a better tool for internal issue creation.

Checklist

• [X] I have read the contributing guide and understand what's expected of me
• [X] I have signed the CLA
• [X] This PR includes tests, or tests are not required/relevant for this PR
• [ ] I have run changie new to create a changelog entry