655: Fix collection fetching for media_collect()

655: Fix collection fetching for media_collect()

The problem is:

    Collection.query.filter_by(id=X, ...)

1. X = form.collection.data
   This works nicely for the completely empty form (X = None).
   It does not work for a selected collection, because X
   will be the collection, not its id.

2. X = request.form.get('collection') (old code).
   This one works mostly, except for the completely empty
   form on postgres, because in this case X = u"__None" and
   postgres does not like comparing an integer column with
   a string.

Fix:
    collection = form.collection.data
    if collection and collection.creator != request.user.id:
        collection = None