Color contrast - Assignment Create/Edit page

Fixes: CNVS-15578

Couple of small tweaks to increase foreground-
background contrast in the Assignments create
UI. Added new variable for Canvas secondary-action
links that contain only an icon glyph (mostly
Close links in Canvas).

IMPORTANT: This ticket is only for the high-
contrast UI. To enable the high-contrast UI, go
to Settings then scroll down and make sure
"Use High Contrast Styles" is set to ON.
Test plan:

- Enable high contrast UI in Canvas (see above)
- Go through each issue Nathan entered into the
  JIRA and compare with the updated UI to make
  sure it's fixed.
- The webAIM color contrast tester can be found
  here: http://webaim.org/resources/contrastchecker/

Change-Id: I5655801f7b8f2060c763701a2857118ed7718e1d
Reviewed-on: https://gerrit.instructure.com/45428
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>