featured images - add support for large image

add page_header image size
add page_header to page
featured photo - move above .site-content

move featured photo above .site-content so it can be full-width
header - tweak on small screens

center nag button on small screens
header - split styles into separate SCSS stylesheet

split styles into separate SCSS stylesheet
make page full-width on narrow (<42em) devices
home - add buttons

add .home-graphic using flex box
tweak caption appearance