UP-4337: Complete the Dynamic Respondr Skin portlet

https://issues.jasig.org/browse/UP-4337

Kept the primary and secondary color choices, but removed the third color from the picker.

Added color selection to:
- Link text
- Footer text color
- Footer background
- Portlet border color (header and body)

Modified the appropriate *.LESS files for color updates.

Added these color preferences to portlet.xml, along with adding the appropriate labels in Messages.properties (English).

Rearranged the Dynamic Respondr Skin portlet into two columns, three colors each side (more room for labels)

This changeset continues to model skins as a collection of portlet preferences.  I'm not going to stand in the way of a big of progress, but I'm continuing to wish that the product were building out a data/object model for skins, a real API for this. ( [UP-4306](https://issues.jasig.org/browse/UP-4306) ).