Flexibly configure base url

[rossbar]

An important component of review workflows is documentation site previews. A common pattern that we have relied on is uploading static builds as artifacts on pr opening/pushes. We were attempting to port this pattern to a mystmd-based workflow for tutorials and ran into a minor issue.

The CI provider (circleci in this case) injects workflow ID metadata in the URL where the artifacts are located. This results in HTML not rendering properly, presumably because the baseurl paths for the artifact preview are different than expected.

Proposal

I'm not sure what the best/easiest solution is here, but it'd be nice to work out a workflow to allow hosting properly-rendered html for PR review (having a solution that doesn't involve netlify would be a plus!). I fully recognize that the pattern described above may be an anti-pattern especially for the new mystmd workflow! Any/all ideas welcome! Pinging @Carreau as well.