.gitignore update for generated files in _site/grunt
Changes to keyboard navigation behaviour on tab controls.

- Keyboard navigation (tab press) through a list of tabs now exposes any hidden tabs as part of stepping through the list
- The 'dropdown' sub list is not given keyboard control as this is visual duplication and not revelant to keyboard navigation or screenreader users.
Force auto sizing on selected tab items.
Improvements to tab size calculation and visibility control. More logical ordering of tab control methods for readability.
Improvements to tab visibility calculation when switching tabs.
Vertical positioning of 'more tabs' dropdown.
Tidy logic around getting and setting tab states when changing selected tab.
Compiled versions.
Improvements to tab dropdown list cloning (remove inherited aria attributes, set aria-hidden). Set dropdown trigger link to aria-hidden.
Compiled HTML updated.
Version number and rc flag
Ensure long tab titles don't overflow their container on smaller viewports.
Update compiled versions