NEW: Allow configuration of default embed width for images and media

Moved 'sensible embed size' logic from JS to PHP to allow setting
through config API
Changed default alignment of images/media to 'Centered, on its own'

More useful default for common usage and responsive images…? In my own
experience this is more practical as I have seen many users embed
images aligned left with text-wrap (without realising what they were
doing), only to then try to resize the image to fill the whole width
which can leave a few words awkwardly positioned next to the image