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 #2916 from EricDunsworth/v3.1-broken-link-fixes

Fixed various broken links, resolved duplicate ID issues, and re-added add a missing image.
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.