Run unit tests + flake8 for community-contributed PRs

[drewbanin]

Describe the feature

Let's tighten the feedback loop for community contributed PRs by running unit tests and flake8 for PRs. We'll still need to kick off integration tests manually, but this will be a meaningful improvement to the contribution flow to dbt.

There's an example of how to implement a flow like this for CircleCI here: https://circleci.com/blog/triggering-trusted-ci-jobs-on-untrusted-forks/

Who will this benefit?

Community contributors