Improved selection interface for advanced classes in page builder

[jeremyfelt]

From @jeremyBass in the original page builder repository:

It was talked about in the open labs that it would be good to have something like stackoverflow tag handling in order to ensure the right classes are used for the spine. I have great ease of use with http://harvesthq.github.io/chosen/#selected-and-disabled-support

This should allow autocomplete for the list of classes (which could be grouped by type when pulled in to autocomplete), but still allow for custom classes to be used.

The whole thought is to reduce trouble shouting, and if it needs to be a feature that is for more then WSU, then the lists could be optional in the admin maybe.

Totally agree. We need to gather a list of the classes that could be applied to section elements. We'll also need to continue to allow for arbitrary class entry.