Rethink how we provide a list of variables to external users

[anth-volk]

At the moment, at https://policyengine.org/us/api, we provide a filterable list of variables that external users (particularly household API users) might want to interface with. We sort these by their label, but don't do much else to filter. This presents many issues:

• On the US, we have many thousands of variables
• We have an entire series of "Abolish..." variables that just neutralize a program, but because their labels all begin with "Abolish," they're very high up in search results, even though the majority of users won't need them
• We don't do anything to separate top-level programs ("CTC") from their sub-components ("CTC maximum amount (adult dependent)")
• We don't define what parameters or variables are
• We lump parameters and variables together, even though they're handled in very different ways when composing an object to POST to the household API