improve navigation flow for custom dashboard_url, fixes CAT-163

test plan:
- in rails console:
-- a = Account.default
-- a.settings[:dashboard_url] = 'http://www.hackaday.com'
-- a.save
- go to canvas root url and log in
- notice that you're taken to the canvas dashboard
- notice that any dashboard links (e.g. the canvas logo at left end of
  header) point to your custom dashboard_url

Change-Id: I28bbbf6d39cace5d9203fadf60b245b83786c0e4
Reviewed-on: https://gerrit.instructure.com/34317
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Houle <nhoule@instructure.com>
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>