Separate EntityMapper from Crayfish-Commons

[whikloj]

We need to require Crayfish-Commons for Islandora because of the EntityMapper. But that means we are intermingling two instances of Symfony and can cause issues like Islandora/Crayfish-Commons#59

We should split the EntityMapper out, require it in Islandora and Milliner (the only places it's needed) and then upgrade Crayfish to use a recent version of Symfony.