realm creation flow: Allow signups from all emails.

This (1) removes the check on whether the domain of the email matches
the Realm.domain of an existing realm and (2) avoids setting `realm =
get_realm(domain)` in the realm creation flow, which would cause the
wrong code path to be followed in the event that the domain in a
user's email address happens to match a deactivated realm.

zerver/forms.py

@@ -171,14 +171,6 @@ class RealmCreationForm(forms.Form):
             del kwargs["domain"]
         super(RealmCreationForm, self).__init__(*args, **kwargs)
 
-    def clean_email(self):
-        # type: () -> text_type
-        data = self.cleaned_data['email']
-        domain = split_email_to_domain(data)
-        if (get_realm(domain) is not None):
-            raise ValidationError(mark_safe(get_registration_string(domain)))
-        return data
-
 class LoggingSetPasswordForm(SetPasswordForm):
     def save(self, commit=True):
         # type: (bool) -> UserProfile

zerver/views/__init__.py

@@ -118,11 +118,15 @@ def accounts_register(request):
         # realm.
         domain = prereg_user.realm.domain
         realm = get_realm(domain)
+    elif realm_creation:
+        # For creating a new realm, there is no existing realm or domain
+        realm = None
+        domain = None
     elif settings.REALMS_HAVE_SUBDOMAINS:
         subdomain_realm = resolve_subdomain_to_realm(get_subdomain(request))
         domain = resolve_email_to_domain(email)
         domain = subdomain_realm.domain if subdomain_realm else domain
-        if (not realm_creation and completely_open(domain)):
+        if completely_open(domain):
             # When subdomains are enabled and the user is registering into a
             # completely open subdomain without going through the correct url
             # for the completely open domains.