UX guide restructure

The goal of this MR was to update the current UI guide to more of an UX guide (starting to detail when to use certain controls and patterns), and add structure to help organize the information. This initial version of the UX guide also lays down the groundwork for principles and personality. This guide is currently documenting the existing design, and will continue to be updated as the design evolves.

Some inspiration/references used:
- https://medium.com/ge-design/ges-predix-design-system-8236d47b0891#.nkf0chj8r
- http://uxcellence.com/2015/11/04/product-principles/
- https://blog.asana.com/2013/10/design-principles/

See merge request !7142