Move primary and user navigations into partial files

Addresses: DES-167

For the upcoming K-12 header, we will be using the K-12
feature flag to split up the markup for the header and
its navigations. This would make the application.html.erb
file a big mess of logic; so to make it more readable, we
will start by putting the primary and user navigations
into partial files.

Code review test plan:
- Make sure the partials are correctly called into the
  application.html.erb file.
- Do the QA test plan.

QA test plan:
- Click on the links in the primary and user
  navigations, and make sure they're all going to
  the right places.

Change-Id: I666d515793b4dcd18838219094f2457f326e6e75
Reviewed-on: https://gerrit.instructure.com/40167
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: Chris Hart <chart@instructure.com>