Fix (and add unit tests for) loading configuration from a file, make HOTPOTATO_TROUBLECODE_URL optional

There are gotchas when using configparser, which I have documented in the code. Basically, section names are case insensitive, while section item keys are always lowercase by default. I missed a few places where I should have made it lower case, and another few bugs snuck in, since the Docker environment is now using environment variables, and there were no unit tests for loading from a configuration file. I've fixed both of these issues here.