chore(lightbox): replaced theme by another lighter

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

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

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

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

File upload form in embed component was overflowing
Merge pull request #6504 from sembrestels/colorbox-theme

Colorbox theme