Quiz Page Contrast Fixes

Fixes: CNVS-15518

Notes:
- increased contrast of 'correct' and 'incorrect' arrows
- updated sprite
- made all text meet contrast requirements
- made published button opaque

Test Plan
- log in as admin
	- navigate to quizzes
	- open a published quiz
		- verify contrast of published button

- log in as student
	- navigate to quiz
	- start quiz
		- answer questions incorrectly and correctly
		- leave some blank
		- verify contrast

Change-Id: I1337d6e9e4cfe08ffbc8a1f013b176ee16e8f246
Reviewed-on: https://gerrit.instructure.com/43068
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>