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')

tests/app/main/views/test_register.py

@@ -1,4 +1,6 @@
 from datetime import datetime
+from bs4 import BeautifulSoup
+from unittest.mock import ANY
 
 from flask import (
     url_for,
@@ -43,11 +45,12 @@ def test_register_creates_new_user_and_redirects_to_continue_page(
                  'password': 'validPassword!'
                  }
 
-    response = client.post(url_for('main.register'), data=user_data)
-    assert response.status_code == 302
-    assert response.location == url_for('main.registration_continue', _external=True)
+    response = client.post(url_for('main.register'), data=user_data, follow_redirects=True)
+    assert response.status_code == 200
+
+    page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+    assert page.select('main p')[0].text == 'An email has been sent to notfound@example.gov.uk.'
 
-    from unittest.mock import ANY
     mock_send_verify_email.assert_called_with(ANY, user_data['email_address'])
     mock_register_user.assert_called_with(user_data['name'],
                                           user_data['email_address'],
@@ -55,6 +58,15 @@ def test_register_creates_new_user_and_redirects_to_continue_page(
                                           user_data['password'])
 
 
+def test_register_continue_handles_missing_session_sensibly(
+    client,
+):
+    # session is not set
+    response = client.get(url_for('main.registration_continue'))
+    assert response.status_code == 302
+    assert response.location == url_for('main.show_all_services_or_dashboard', _external=True)
+
+
 def test_process_register_returns_200_when_mobile_number_is_invalid(
     client,
     mock_send_verify_code,