css: Adjusted table button margins

... again!
css: Added margin-top to flight details table

Fixes the duplicate border below the tabs
css: Use icons for comments and stats on all resolutions

This scales better than using text, which can be problematic due
to i18n issues.
css: Updated FontAwesome to latest v3.2.1
flights/details-table: Use FontAwesome icons instead of unicode symbols
css: Extracted bootstrap-visibilities.css
css: Simplified responsive inline element hiding

Replaced .hidden-inline-* by the standard .hidden-* and added .visible-*
styles