fix custom dashboard_url during cc#confirm, fixes CAT-126

also ensure it doesn't asplode if there's no @current_user

test plan:
1. set a (valid) dashboard_url on your root account
2. register as a new user (via fft form or whatever)
3. follow the link in your email to set a password
4. confirm you are redirected to the custom dashboard after submitting the
   form
5. confirm that it carries along your user id

Change-Id: Iab22494b394cbccecd5097ad30dbf933e7949e10
Reviewed-on: https://gerrit.instructure.com/33824
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>