Get the patterns and occurrences related to a corpus

'/corpus/:corpus' (and '/item/:corpus/:item') may also return patterns and their occurences.

The code was functional in Cassandre 2 (using the "join proxy" to join patterns and related KWIC) but it got broken in Cassandre 3.

Working examples are available on cassandre.hypertopic.org (try `enseignants-decrocheurs` as the corpus). 