Move bills to the Pro Publica Congress API

[konklone]

This moves all of the bill-fetching code to use the Pro Publica Congress API. There should be no remaining areas where we hit the Sunlight congress API's /bills endpoint.

It also moves a couple of straggling Legislator-fetching endpoints over.

There are still a few outstanding issues before this is mergeable and/or releasable:

• [x] Re-enable pagination. It's been disabled while @dwillis works out some API instability issues that seem to be generated most acutely when using offset=0 for the first page of results. Right now all bill listing requests only fetch page 1 (no offset parameter sent), no matter what page the code is requesting. This must get restored before shipping, as it's clear breakage right now.
• [x] I'd like to get clarity on the sort order of a few bill listings, since they are relevant to how users get push notified. I've asked in https://github.com/propublica/congress-api-docs/issues/68.
• [x] Search results are by relevance only, not recency. Ideally, PP will implement both options as discussed in https://github.com/propublica/congress-api-docs/issues/74, and we can go back to the way things were before. If not, then I'll need to at least deactivate, if not delete, the push notifications tied to recent-bill-search.
• [x] I've moved the Active bill feed to use the updated list in the PP API, but the signal-to-noise ratio is much higher here than before. Since the PP API does have a concept of (and flag for) whether a bill is active, I've requested a filter and feed for active bills in https://github.com/propublica/congress-api-docs/issues/71. I'd ideally like to see this ship and then move the app to use it, since the signal-to-noise ratio is relevant to how users get push notified.
• [x] Come up with reliable unique IDs for bill actions, since we were using time and now we only have date (so they will definitely collide). PP has responded to https://github.com/propublica/congress-api-docs/issues/75 and added a predictable id field to actions in the order they are made, which will let us proceed.
• [x] Create reliable unique IDs for bill votes by moving voted_on back to voted_at by parsing the time field available on bill votes. [Edit: Ended up doing this by appending the roll_id to the bill_id. This works for as long as votes are always roll call votes (which is the case in the PP API).
• [x] Test out moving from the previous release APK to a new release APK, where we had existing notifications existing starred legislators and bills, and make sure the new APK reads from the old data correctly.

[konklone]

Whew! Finally, legislators and bills are now completely moved to the Pro Publica API.

Everything ported, including bill searching. I didn't have to remove any features. I tested out a migration path with legislators and bills starred, and various notifications turned on, and the migration worked without crashes or bugs.

Where there is an issue that I have to consider (in a future issue/PR) is how to handle push notifications users are due to get. Some will be from the fact that the PP API differs somewhat for some endpoints, so search results and feeds will show some new information. That's okay (though still undesirable). Less okay is that all bill actions and bill votes will get falsely notified as new, because the ID format changed (away from a timestamp and over to an ID-based scheme).

What I may want to do is have a post-install hook that does a quick scan for all (or some?) existing notification subscriptions, and marks everything as "seen" without notifying the user. There's no feature for post-install hooks in the app, but I could add one if I wanted.

In any case, this is still releasable if I wanted to, so, merging and moving on.