Crash on confirming non-existing registration entry

Ghost User
2018-08-07T12:34:00+02:00

Closes #665

#### Previous behaviour
Steps to reproduce:

1. Go to `/registration/confirm-email/00000000-0000-0000-0000-000000000000`
2. :boom:

#### New behaviour
Steps to validate that it works:

1. Go to `/registration/confirm-email/00000000-0000-0000-0000-000000000000`
2. Redirect to registration page