feat(ci): replace test app w/ browsable artifacts

This deprecates the previous implementation of test apps with a dynamic deployment in a git-push-PaaS.

It reuses automatic, browsable deployments of GitLab CI artifacts with GitLab Pages, which is a welcome side-effect of using the prior when the latter is present.

References:

• https://k33g.gitlab.io/articles/2020-07-23-GITLAB-PAGES-EN.html
• https://stackoverflow.com/questions/55596789/deploying-gitlab-pages-for-different-branches/58402821#58402821
• https://docs.gitlab.com/ee/ci/yaml/yaml_optimization.html#configure-your-ide-to-support-reference-tags

Closes #59 (closed)