Improve get_tracker_lists

[boolean5]

A couple of suggested improvements for get_tracker_lists:

1. Raise a ValueError for invalid categories found in the configuration file, just like we do for tags.

2. If the categories key of a configuration section exists but is empty, get_tracker_lists passes to get_domains_from_category_filters a category_filters list that is equal to [""].

   This results in the DisconnectParser raising a KeyError when get_domains_from_category_filters tries to get the domains with category "".

   To avoid this, we could either raise a ValueError when we encounter an empty categories key in get_tracker_lists or fall back to using the DEFAULT_DISCONNECT_LIST_CATEGORIES, just like when there is no categories key.

[boolean5]

Regarding (2), after discussing it with @englehardt and @say-yawn we decided to go with raising a ValueError, not only when the categories key exists and is empty, but also when it does not exist. We will get rid of the DEFAULT_DISCONNECT_LIST_CATEGORIES, which can be confusing, and require explicitly determining the categories in every configuration section.

For this, we will first need to adjust the configuration files in https://github.com/mozilla-services/shavar-list-creation-config/, as described in mozilla-services/shavar-list-creation-config#64.