Make hover/focus/active states consistent for dropdowns

We currently have several styles for dropdowns. These three different versions are taken from the Issues list page:

The dropdowns will all use fa-chevron-down from Font Awesome. The border and chevron will change color for hover, focus and active states.

The following table shows the colors to be used for each state:

The text color to be used for each type of dropdown is as follows:

Edited Jun 19, 2017 by Pedro Moreira da Silva