further full-screen mode optimizations

[eplebel]

Details:
Different settings for HTML vs. PDF links (the former which naturally/typically scales nicely to even the smallest 320px mobile screens!)

• HTML:
  • display full-screen icon on all viewport sizes, but:
    • viewports <1300px: Full-screen view ONLY (fancyBox viewer!)
    • viewports >1300px: Full-screen view (fancyBox) AND Multi-panel VIEW available
• PDFs:
  • viewports < 360px: hide full-screen icons because insufficient width to read a PDF, even if viewing in "landscape" orientation (whereas "medium-sized" mobile screens (eg s5 or iPhone 6 and up) DOES offer sufficient width in landscape orientation for PDF article reading)
  • viewports 360px to 1300px: Full-screen view ONLY (fancyBox viewer!)
  • viewports >1300px: Full-screen view ONLY (fancyBox) AND Multi-panel VIEW available