chore(lightbox): replaced theme by another lighter

Using colorbox example 4: http://www.jacklmoore.com/colorbox/example4/

Fixes #6475
chore(ui): using elgg ajax throbber in lightbox

Fixes #4592
fix(ui): CKEditor was weird in lightbox

lightbox is not in .elgg-page and css wasn't overriding.
fix(ui): make lightbox scrollbar look better

File upload form in embed component was overflowing