Refactor mix environment files so they are untethered from deployment configurations

# Problem

Per some advice from @DavidAntaramian:

> In essence, your Mix environment is not the same as a Rails environment. Very different beasts. The Mix environment should only control build concerns (like, "Should debug information be stripped from this file?") not runtime concerns. We've been bad as a community in communicating that, though.

> So, for example, the URL for the web app used in emails, that should really be stored in an environment variable. You might provide a default in the `config.exs` file, but then try and load the OS environment variable inside the application's `start/2` function and inject it into the application's Erlang environment store (`Application.put_env(:code_corps, :allowed_origins, allowed_origins)`)

> Ecto and Phoenix are trying to be better about allowing this type of thing. So Ecto has a callback on Ecto.Repo called `init/2` that you can define and use to fetch environment variables defining connection information rather than storing that in the config file (https://hexdocs.pm/ecto/Ecto.Repo.html#c:init/2). Phoenix.Endpoint has a similar `init/2` function for defining URL and HTTP transport configuration (https://hexdocs.pm/phoenix/Phoenix.Endpoint.html#c:init/2)

This is going to require:

- cleaning up the config files
- setting the new configuration on the servers, where needed
- documenting the configurations somewhere so they are not solely stored on our servers (or with any specific PaaS)