Accessibility tweaks to platform

Accessibility tweaks:
- make focus state of primary buttons more pronounced
- increase contrast of submission_feedback text
@caesar2164, nice job pulling the focus state out from the hover/active ones. This is a good call.

Previewing this locally, I'm a bit concerned that the solution here is:

1. scoped to just these types of buttons versus a standard focus state/styling that's consistent for all buttons (and perhaps actions including links, form fields, etc.). Being consistent would help zoomed in users become familiar with similar UI elements and their focused state as they interact more and more with our platform's UI.

2. not bulletproof when it comes to color contrasting - currently in my edX centric colors provide enough contrast when they are swapped (using the properties you're setting), but we can't promise that these Sass variables will always have enough contrast ratio on other platforms.

- - -

With that said, I'd suggest focusing on the outline or box-shadow properties as good ones that we can make consistent on any UI button and can be more divorced from the bg/color of the element itself. Make sense and are you cool with trying that alternative out here?