Fixes default opacity on buttons

Testing procedure:

One of the areas we saw it most apparent is in the communications
area.

http://screencast.com/t/SYJYGKVuj

These should now be visible, but just look disabled.

fixes: CNVS-14572

Change-Id: I5bc9854c59b87ba539e69ff3e066243243197f10
Reviewed-on: https://gerrit.instructure.com/38652
Reviewed-by: Vincent Gonzalez <vgonzalez@instructure.com>
Product-Review: Vincent Gonzalez <vgonzalez@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>