change "Twitter Share Button" class name

Official Twitter share button started using `twitter-share-button` for their
class names.
add "Share Buttons" widget

This replaces "Twitter Share Buttons".
optimize `figcaption`'s `top` value
remove "Site Navigation" active border
fix "Share Buttons" layout

"Share Buttons" should not wrap on small screen.
increase "Searchbox" width on small screen
version 6.43.5