This adds a job to test the UI on CircleCI, including the sort of branch
pattern-matching from #5839, so .-ui/ branches only have that job
and not the non-UI ones.

I considered having an entire workflow for UI, which could have separate
jobs for linting vs Ember tests, but the lint commands take so little time
that it didn’t seem worth it.

There’s no use of nvm to change the Node version as the Docker image
is what controls that. It’s annoying to have to update the version in multiple
places, but probably infrequent.