No more textures in Canvas

fixes CNVS-15254
fixes CNVS-14958

This takes out the big textured backgrounds we have in Canvas and
makes everything flat. I've attached screenshots to the story to
see what all is being changed. This affects the following areas:

- Main global header
- Main sidebar left nav
- Body background

This also takes off some annoying drop-shadows we had on the user
navigation (top right) and the global Courses drop-down box.

Yay for a cleaner looking Canvas!

Testing:

- Load up Canvas, make sure the color changes appear for you and
match the screenshots

Change-Id: I3557e66d7d91c3999ad67bfa92202e61c7da44cb
Reviewed-on: https://gerrit.instructure.com/40527
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>