Update config from list of variables to class

[lucas-wilkins]

Use a class to specify the config, so its clear what variable should and shouldn't be there. It's hard to keep things tidy when you can't easily trace usages.

[lucas-wilkins]

Step 1: Move config stuff (custom_config, local_config, LocalConfig and their copies) into a single class that can be inspected
Step 2: Make config IO work with yaml, NO OUTPUTTING PYTHON CODE
Step 3: Remove any usused config items
Step 4: Relocate any items that don't belong
Step 5: Make config more of a data container by removing any methods and dynamic stuff
Step 6: Rename config items in a consistent way
Step 7: Add schema for validating config files

[lucas-wilkins]

TODO: images should be located using importlib, not through ad-hoc dynamic paths in config