Add a GitLab CI workflow and test compatibility with Canvas

Problem/Motivation

After the snafu of 🐛 White Screen of Death with Canvas Active , we really need to make sure that this theme doesn't break stuff inadvertently. To that end:

• Let's implement a bog-standard GitLab CI workflow, based on the DA's template.
• And let's add a simple functional test that enables this as the default theme, then installs Canvas.