ACTION-297: Fix keyboard focus of lightbox functionality

From Kevin White:

For example log in box, and information panels on homepage. When the Log In link is activated with the keyboard, the lighbox pop-up appears but keyboard focus remains on the Log In link. Tabbing will simply take the user to the next link in the toolbar. This makes it nigh on impossible to access the login lightbox. This is particularly problematic when a screen reader user activates a homepage link that requires login as the activation will cause no apparent effect as the popup will not be available.
Suggestion: When activated the Log In link should shift focus to the lightbox. Ideally focus should be contained within the lighbox until it is closed, at which point it should return to the Log In link. This approach should be adopted for all lightbox popups.