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`.
catch exceptions here. Flask might handle this in a reasonable way - 500 response returned if an exception is raised while fulfilling the request. Does that result in an ok user experience? Make sure it's not exploitable. Redirecting back to the col page is probably right.