Fix 500 error if revisiting registration page

If you go back to this part of the registration flow then you get a 500
error, because we’re relying on something in the session. We clear the
registration info from the session after you’ve registered successfully.

Also there were no tests for the happy path of this page either
¯\_(ツ)_/¯

app/main/views/register.py

@@ -89,4 +89,6 @@ def _do_registration(form, service=None, send_sms=True, send_email=True):
 
 @main.route('/registration-continue')
 def registration_continue():
+    if not session.get('user_details'):
+        return redirect(url_for('.show_all_services_or_dashboard'))
     return render_template('views/registration-continue.html')