[CLC-1081] Show remainder of feed when Google service unavailable

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-1081

On top of hardening the merged models handling google endpoints, did some refactoring with the GoogleProxy to clean up the complexity a bit. Most of the items that deal with the api_client have been moved to a separate class. The page fetching is now based on a lazy enumerator instead of a simple array model to 1) ease memory use, and 2) merging code can now walk over the pages and terminate subsequent calls prematurely if needed. Also wired in the ability to pass the fakeable_proxy some options (to ease the recording matching)