Needs testing

According to Testing Flask Applications,

Something that is untested is broken.

So let's add some tests!

Current status (!31 (merged)):

• We have a basic .gitlab-ci.yml.
• It doesn't really test anything, but at least it runs pycodestyle as a linter on main.py.
• It also generates a container, based on Google's SDK base container, that includes pycodestyle as well as the requirements.txt for Python 3.
• Enable the Google Cloud Datastore emulator to run as a GitLab CI "service". Maybe we should build a special container image with gcloud beta emulators datastore start as the entrypoint and exposing port 8081. In this case, we should try to be smart about building multiple container images.
• Actually write some tests—see Testing Flask Applications. I probably need to learn how pytest works.