UI: file list + preview added - "PrimaryDataset"

* preview functionality added for “.txt” files
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)