Email validation link lost after login

[pmeulen]

The email validation in SelfService works by sending a mail to the user's email address with a link in the form: https://sa.test.surfconext.nl/verify-email?n=337c06301b71d9ed4e2452969d737961

Steps to reproduce:
0. Login to selfservice with an account from an institution with email validation enabled (e.g. joe-a1)

1. Add a new token
2. Open a new private window or otherwise remove the selfservice session cookie and open the verify-email link in that window
3. You need to authenticate, if asked use the same account (joe-a1)

Expected behaviour: The email address is validated and the user arrives at e.g. the https://sa.test.surfconext.nl/second-factor/ page. Selfservice remembers the link that was used before started the SAML authentication and redirects the user back to that link. This must happen for all links that trigger a new authentication: both when no session exists and when the session is expired. (#360)
What happens: After authentication the user is directed to the home page (https://sa.test.surfconext.nl/overview). The link is lost. This is confusing users who click the link in the email and are taken to the selfservice homepage as nothing happens.

selfservice version: 5.0.4p1

Please add tests for this scenario.

[pablothedude]

Seems related to: #336
The related Behat test in devconf: OpenConext/OpenConext-devconf#17
We should maybe also validate other components with email links and other get params.