Issues/464

This is branched from https://github.com/freedomofpress/securedrop/pull/463, and resolves #464.  This adds filters for the journalist interface, both the overview of sources and list of documents.  It also makes sure that any souces / submissions that are not visible will not be deleted/starred/unstarred.  Only visible items will be acted upon.  This preserves the graceful degradation of both `index` and `col`.
Not a requirement for merging, but would be nice: I think we can condense this into a single database query, since the Submissions table has all of the necessary info that we're currently getting from `os.stat`.