Migrate committee information to the Pro Publica Congress API

[konklone]

This moves the committee endpoints entirely over to the Pro Publica Congress API.

Some notes on changes in client behavior:

• We can't display the title (e.g. Rep.) of members of Congress on joint committees, as the legislator's chamber is not inferrable when looking at committee memberships for joint committees. (See: https://github.com/propublica/congress-api-docs/issues/140)
• We now don't have a way of showing Vice Chairman of some joint committees, and instead show a ranking member. (See: https://github.com/propublica/congress-api-docs/issues/142)
• We don't have any concept of Ex Oficio members or anything else other than chair and ranking member. Those are the fields standardized for all committees across the Pro Publica API.
• Senate subcommittees are missing, but it looks like this will be fixed server-side. (See: https://github.com/propublica/congress-api-docs/issues/144)
• Some committee ordering is out of whack, due to null rank fields on some joint committees. (See: https://github.com/propublica/congress-api-docs/issues/141)
• We no longer have two joint committees which only sort of operate as such, one on Europe and one on narcotics control. (See: https://github.com/propublica/congress-api-docs/issues/135, https://github.com/propublica/congress-api-docs/issues/138)
• We're relying on committee IDs being 4-character subsets of subcommittee IDs when parsing subcommittee memberships of individual members on their details endpoints. (See: https://github.com/propublica/congress-api-docs/issues/126)

Any changes or fixes for the above will happen server-side, so this is stable to ship from the client perspective.

@dwillis - CCing you in case you're interested in reviewing the client code.