fix masquerading for accounts with custom dashboards

fixes CAT-637

Test plan:
 1. Set a custom dashboard for an account:
    a = Account.find(x)
    a.settings[:dashboard_url] = 'http://some-url'
    a.save!
 2. Clicking the logo in the top left should still take you
    to your custom dashboard, however..
 3. Clicking "Become" on a user's profile page should start
    a masquerading session

Change-Id: Ibe60f2a6fb80b21ca58251f3f02591c1bb7ec9b6
Reviewed-on: https://gerrit.instructure.com/43559
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jeff Belser <jbelser@instructure.com>
Reviewed-by: Jeff Belser <jbelser@instructure.com>
QA-Review: Jeff Belser <jbelser@instructure.com>
Tested-by: Jeff Belser <jbelser@instructure.com>
Product-Review: Adam Phillipps <adam@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>