Make domain name configurable

[senderle]

Right now, the traefik.toml file has a ton of hard-coded references to the domain name. And at least one of our python scripts also has the domain hard-coded. That's no good. It should be an environment variable.

The python scripts are easy because it's just a matter of getting the value of an environment variable (and making sure that docker-compose.yml loads the relevant file from .envs in the text_to_md and voyant_gen services so that the environment variable is actually available). Getting environment variables in python is easy: https://stackoverflow.com/questions/4906977/how-to-access-environment-variable-values

Figuring out how to fix this in traefik.toml is way more annoying. From what I can tell, Traefik doesn't yet do environment variable interpolation in its config files. There is an issue about this here from 2015. It's their seventh issue out of like five thousand. It was just closed 15 days ago, and so there is presumably now support for this in the bleeding edge version. But we probably don't have the bleeding edge version.

I am not entirely sure how we should handle this part! We might be able to upgrade to the bleeding edge version and hope nothing breaks; we might just wait until this feature makes it into a more stable version; or we might come up with some stupid ad-hocery that works OK.

[senderle]

This isn't easy but it's important so I'm marking it as high priority.

[senderle]

Also, to get #7 working for real, this will have to be solved too. So I'm assigning it to you as well @Vicenteguallpa.