ATLAS: initial release of ATLAS sections

* Adding ATLAS sections.  (PR #399)
ATLAS: initial release of ATLAS collections

* Amends ATLAS collection tree ancestors and introduces `ATLAS Derived
  Datasets` collection.  (PR #399)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
UI: updates breadcrumbs + font changes

* updates how text look across-site. more narrow
header/search-bar/breadcrumbs
* adds drop down button to breadcrumbs to browse throughout experiments
* removes rhs “choose experiment” pills
UI: updates "About CMS Physics" pages

* gives similar layout to “Get Started” and “About” pages