Fixed broken Terms and conditions links and added rel="external" to PHP variant links.
Merge pull request #2915 from EricDunsworth/v3.0-broken-link-fixes

Fixed broken Terms and conditions links and added rel="external" to PHP ...
Icons: Add dual icon for external download

Closes gh-2248
Merge pull request #2917 from nschonni/external-download-icon-fix

Icons: Add dual icon for external download
Tabbed interface: improvements for screen readers

 1. Removed `enter` key behaviour for tabs as it was preventing a
    smooth transition to/from forms mode which resulted in an extra
	`enter` key press being required.
 2. Set `aria-live="off"` for the tablist.  This greatly reduces
    the amount of content read by screen readers when the arrow
    keys are used to change tabs or `escape` is used
    to stop the tab cycle.
 3. Set `aria-live="polite"` on the "Play" button.  This causes
    changes to this buttons text to be spoken.
 4. Backported `escape` key to stop tab rotation.

Discussion is in #1081.
Slide out tab SCSS: Restored "raw" content in small/medium views.

Closes gh-2913
Resolved merge conflicts.
Merge pull request #2920 from patheard/tabs-aria-live

Tabbed interface: improvements for screen readers
Resolved merge conflicts.
Resolved merge conflicts.
Tabbed interface: fix set of tablist aria-live attribute
Merge pull request #2925 from patheard/tabs-aria

Tabbed interface: fix set of tablist aria-live attribute
GC Web Usability theme: Moved definition of swapPNG function so in scope of all calls to the function (fixes #2933).
Merge pull request #2934 from pjackson28/fixswappng

GC Web Usability theme: Moved definition of swapPNG function so in scope of all calls to the function (fixes #2933)
Multimedia player: Added label and srclang attributes to caption tracks in working examples (fixes #2935).
Merge pull request #2936 from pjackson28/updatemplayerexamples

Multimedia player: Added label and srclang attributes to caption tracks in working examples (fixes #2935)
Multimedia player: Corrected incorrect data-type on English captions and indenting issue in French.
Merge pull request #2938 from pjackson28/mplayerfixes

Multimedia player: Corrected incorrect data-type on English captions and indenting issue in French
Resolved merge conflicts.
Tabbed interface: fix enter key blocking on tabs

1. `e.preventDefault()` has been removed from the tab's keydown event
   handler.  This allows enter key presses to pass through to screen
   readers that don't implement their own enter key behaviour.
2. The play button only has `aria-live="polite"` set when it has
   focus.  This prevents extra speaking of its changed text.