New Canvas header for K-12

Fixes: CNVS-14508

Adds new markup and styles for K-12 Canvas header.
Big change to application.html.erb - the #header
is split into 2 sets of markup: the new K-12
markup and the existing Canvas header.

To enable the K-12 feature flag:

- In Rails Console, enable the k12 feature flag
  as follows: Account.find(1).enable_feature!(:k12)

Note: Due to Canvas caching, the Courses link in
the primary nav will only update once every 15 mins.
To update the link, either wait out the cache or
temporarily remove the cache code around the list
item element.

Test plan:
- Make sure the K-12 feature flag is disabled.
  To disable: Account.find(1).disable_feature!(:k12)
- Click around the existing Canvas header. Make sure
  it is working as expected in both regular and
  high-contrast modes.
- Enable the K-12 feature:
  Account.find(1).enable_feature!(:k12)
- Click around the new header. It should look like
  this in regular mode...
  http://cl.ly/image/2b1V3e2g0C3z/k12-regular.png
  ...and this in high-contrast mode:
  http://cl.ly/image/3E0x3O2L3P2B/k12-high-contrast.png
- The new header is responsive down to tablet-size.
  When you reduce the screen width enough, the icons
  should stack on top of the link text in the primary
  navigation.
- The new header also features an Inbox link in the
  primary navigation (the existing header does not
  have this). Confirm this is working and that the
  user nav in the top-right no longer has an Inbox
  link.

Change-Id: I38cd0d215fe67000fba01204f208801f550df089
Reviewed-on: https://gerrit.instructure.com/38925
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>