templates: physics objects link in "Get started"

* Fixes physics objects link and edits instructions.
  (closes #198) (PR #201)
templates: physics objects link in About CMS Data

* Fixes physics objects link.  (closes #153) (PR #201)
UI: image renaming

* FIX Screenshot of ROOT browser (closes #202)
UI: use translated collection names

* (closes #206)
UI: records: file desc + preview only with .ig

* templates: real file descriptions (closes #204)
* Event preview in derived datasets shows only if “.ig” files exist
(closes #164)