Redesign login and registration pages.

This completes a major redesign of the Zulip login and registration
pages, making them look much more slick and modern.

Major features include:
* Display of the realm name, description and icon on the login page
  and registration pages in the subdomains case.
* Much slicker looking buttons and input fields.
* A new overall style for the exterior of these portico pages.

zerver/tests/test_docs.py

@@ -37,7 +37,7 @@ class DocPageTest(ZulipTestCase):
             self._test('/hello/', 'workplace chat that actually improves your productivity')
             self._test('/integrations/', 'require creating a Zulip bot')
             self._test('/devlogin/', 'Normal users')
-            self._test('/register/', 'get started')
+            self._test('/register/', 'Sign up for Zulip')
 
             result = self.client_get('/new-user/')
             self.assertEqual(result.status_code, 301)