ATLAS: new name "ATLAS Learning Resources"

* Introduces new collection and file name for ATLAS external (->
  learning) resources.  (addresses #517)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
base: s/external/learning

* Respects recent renaming of "external" resources to "learning"
  resources.  (closes #517)

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