I2PSnark UI bugfixes

- Fix broken collapsible panels issue for browsers that don't support the
  feature by conditionally loading override CSS to expand panels by default
  and disable hover/active states for panel headings (tickets #2002, #2026)
- Add UI option to configuration section to enable collapsible panels, and
  disable the option if a non-compliant browser is detected
- Fix multiple instances of snark refreshing to the homepage (ticket #2028)
  (patch supplied by mindless)
- Tentative fix for caching of images so ajax refresh doesn't reload all
  image resources
- Standardize 'Save Configuration' action to return to top of the page
  (so we can see message log entry)