ATLAS: separate Higgs Challenge collection

* Separates ATLAS Higgs Challenge 2014 collection from the main
  collection tree, and puts all records into this collection only, so
  that the collection can be deployed semi-publicly before the official
  public launch.  (addresses #660) (addresses #724)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
base: better detection of parent collections

* Fixes problems with detecting parent collection IDs for separate
  collections, such as ATLAS Higgs Challenge 2014, now that it has been
  unplugged from the collection tree.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
ATLAS: fix for 856 MARC tag

* Fixes URLs for ATLAS records (`$a` should be `$u`).
  (addresses #660) (see PR #741)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
LHCb: fix for 856 MARC tag

* Fixes URLs for LHCb records (`$a` should be `$u`).

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>