add dashboard_url setting to root accounts

fixes CAT-13

test plan:
- in rails console (for canvas):
-- a = Account.default
-- a.settings[:dashboard_url] = 'http://www.hackaday.com'
-- a.save
- visit canvas root url
-- if already logged in, you'll be redirected to hackaday.com
-- else, log in, then you'll be redirected to hackaday.com

Change-Id: Ib6e48ae9c3be3761810f0e859b441dcc2edd1f23
Reviewed-on: https://gerrit.instructure.com/32011
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeff Belser <jbelser@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>