CMS: new collection of External Resources

* Adds new collection of "CMS External Resources".  (closes #253)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
templates: removal of UserEcho

* Removes User Echo.  (addresses #258)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
templates: about this portal

* Adds text for About this portal.  (addresses #65)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
templates: collection page UI amendments

* Renames `Collection Items` into collection-specific phrase.

* Moves portal box to be displayed after collection name.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
templates: safer display of imprint

* Safer display of imprint that may be missing in some collections.
  Fixes traceback when displaying "CMS External Resources".

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
views: code kwalitee improvements

* Improves code kwalitee of `views.py`: changes indentation from tabs to
  white space and amends PEP8 kwalitee.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
collections: new regular/virtual collection setup

* New collection setup in which experiments are attached as virtual
  collections under Home and dataset/tools/etc collections are attached
  both under their experiments and under Home as direct daughter
  collections.  This eliminates extra level in the collection hierarchy,
  making facets work better.
  (addresses #70) (addresses #175) (addresses #258)

* Amends "For Education" and "Collection overview" pages accordingly.

* NOTE When other experiments join in, we may need to revisit this,
  e.g. introduce separate facets for separate experiments.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
templates: no "lorem ipsum" in collection overview

* Removes "Lorem Ipsum" that was still left on the collection overview
  page.

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