Mediacloud

* added a "modified_before" parameter to DirectoryApi.feed_list for use by rss_fetcher

* ⚠️ 🚧 Complete rewrite. This is first work on supporting new API for cross-platform search and directory of
collections/sources/feeds. This is an entirely new system we are building out.


Version 3
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* add TIMEOUT_SECS property, defaulting to 30 seconds (configurable by user on `MediaCloud` object)

* update MongoDB storage helper to use URI for connection (so it support user/pass authentication)

* prevent people from trying to page through randomly sorted `storyList` results - it doesn't work so we shouldn't
let them try

* pass `old_stopwords` param as int to `storyWordMatrix` and `wordCount`