allow to select corpus by URL

This is the functionality needed to connect the Laudatio Repository with ANNIS. 

A link like https://korpling.german.hu-berlin.de/annis3-snapshot/#c=laudatio2:corpusridgesv1_complete can be used to select the corpus represented by a valid corpus name or external persistant identifier. An instance wide mapping determines which persistant identifier maps to which "real" corpus name.