New :active and :hover styles on Courses navigation

fixes CNVS-15580

We've listened to some of the feedback from our customers and
have tweaked some of the stylings on the left hand courses nav
to make the active and hover states stand out more.

Test plan:

- Go to a course
- The left side nav should now have a slightly darker background,
and active/hover states should be white background with blue link
- High Contrast setting shouldn't change and will have the dark
background on active and hover with white text; disabled links will
now have a gray background with the dark link

Screenshot of new look: http://cl.ly/image/3f0f412R040X

Change-Id: Ibfd504a6e752d640ddfb351967c2287bcbf9c4a3
Reviewed-on: https://gerrit.instructure.com/41240
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Reviewed-by: Vincent Gonzalez <vgonzalez@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>