website: support adding custom site-wide buttons on the right of the breadcrumbs navigation UI
css/main.css: increase the padding on the text buttons to improve UX
css/main.css: make the blue button darker; match the banner colour
css/main.css: set the maximum content width to 800px and center the content.
css/main.css: always show a vertical scrollbar

This is to ensure that all page layout positioning has the
scrollbar accounted for. Otherwise, the layout would bounce when
moving between a page with and a page without a vertical scrollbar.
Merge remote-tracking branch 'template/master'