CLC-3339 Extract AbstractModel superclass, with UserSpecific and SingleSource model subclasses
CLC-3339 Give smart_fetch_from_cache an optional jsonify mode
CLC-3339 Push filtering and JSON caching down into YoutubeProxy layer
CLC-3339 Push formatting functionality down into PlaylistsProxy level
CLC-3339 Adjust expirations for textbooks & video proxies
CLC-3339 Remove MyTextbooks model, inline it into TextBooksProxy
CLC-3339 Fold the Playlist and Youtube models into their proxy classes
CLC-3339 Move BlogFeed into lib/proxies
CLC-3339 Rename BlogFeed -> BlogFeedProxy
CLC-3339 Remove useless SingleSourceModel
Merge pull request #2204 from ctweney/clc-3339-b

CLC-3339 Models not part of Live Updates can cache error states for a long time