Also, keep the html5 boilerplate link in the built CSS.

https://www.youtube.com/watch?v=L-4VxEtWyRo

Thanks to @collinanderson
Closes #14567

Fixes #14557: Reset the bottom margin on .form-group:last-child in .navbar-form to prevent extra spacing

Fixes #14319: Prevent jumbotron reflow by ensuring padding of .container isn't interferred with by .jumbotron side padding

Fixes #14443: Only transition what needs transitioning for thumbnails to avoid Safari/Chrome render bug