Add smooth scrolling

[adamkudrna]

Enable animated scrolling to anchors on current page. Should be probably opt-in (via CSS class on parent element of links to be scrolled smoothly) according to BUI contributing guidelines.

Must support browser history.

Possible solutions:

• https://css-tricks.com/snippets/jquery/smooth-scrolling/ (doesn't require a plugin)
• https://github.com/kswedberg/jquery-smooth-scroll