registration: Use already_registered to show error.

Use this new variable to determine if the user already exists while
doing registration. While doing login through GitHub if we press
*Go back to login*, we pass email using email variable. As a result,
the login page starts showing the "User already exists error" if we
don't change the variable.

templates/zerver/login.html

@@ -92,7 +92,7 @@ common.autofocus('#id_username');
                     </div>
                     {% endif %}
 
-                    {% if email %}
+                    {% if already_registered %}
                     <div class="alert">
                         {{ _("You've already registered with this email address. Please sign in below.") }}
                     </div>

zerver/tests/test_signup.py

@@ -1135,6 +1135,8 @@ class UserSignUpTest(ZulipTestCase):
         result = self.client_post('/accounts/home/', {'email': email})
         self.assertEqual(result.status_code, 302)
         self.assertIn('login', result['Location'])
+        result = self.client_get(result.url)
+        self.assert_in_response("You've already registered", result)
 
     def test_signup_invalid_name(self):
         # type: () -> None

zerver/views/auth.py

@@ -495,6 +495,12 @@ def login_page(request, **kwargs):
     except KeyError:
         pass
 
+    try:
+        already_registered = request.GET['already_registered']
+        template_response.context_data['already_registered'] = already_registered
+    except KeyError:
+        pass
+
     try:
         template_response.context_data['subdomain'] = request.GET['subdomain']
         template_response.context_data['wrong_subdomain_error'] = WRONG_SUBDOMAIN_ERROR

zerver/views/registration.py

@@ -285,7 +285,8 @@ def send_registration_completion_email(email, request, realm_creation=False):
 def redirect_to_email_login_url(email):
     # type: (str) -> HttpResponseRedirect
     login_url = reverse('django.contrib.auth.views.login')
-    redirect_url = login_url + '?email=' + urllib.parse.quote_plus(email)
+    email = urllib.parse.quote_plus(email)
+    redirect_url = login_url + '?already_registered=' + email
     return HttpResponseRedirect(redirect_url)
 
 def create_realm(request, creation_key=None):