Add option for hosting each realm on its own subdomain.

This adds support for running a Zulip production server with each
realm on its own unique subdomain, e.g. https://realm_name.example.com.

This patch includes a ton of important features:
* Configuring the Zulip sesion middleware to issue cookier correctly
  for the subdomains case.
* Throwing an error if the user tries to visit an invalid subdomain.
* Runs a portion of the Casper tests with REALMS_HAVE_SUBDOMAINS
  enabled to test the subdomain signup process.
* Updating our integrations documentation to refer to the current subdomain.
* Enforces that users can only login to the subdomain of their realm
  (but does not restrict the API; that will be tightened in a future commit).

Note that toggling settings.REALMS_HAVE_SUBDOMAINS on a live server is
not supported without manual intervention (the main problem will be
adding "subdomain" values for all the existing realms).

[substantially modified by tabbott as part of merging]

zerver/lib/actions.py

@@ -1929,12 +1929,12 @@ def do_change_stream_description(realm, stream_name, new_description):
                  value=new_description)
     send_event(event, stream_user_ids(stream))
 
-def do_create_realm(domain, name, restricted_to_domain=True):
-    # type: (text_type, text_type, bool) -> Tuple[Realm, bool]
+def do_create_realm(domain, name, restricted_to_domain=True, subdomain=None):
+    # type: (text_type, text_type, bool, Optional[text_type]) -> Tuple[Realm, bool]
     realm = get_realm(domain)
     created = not realm
     if created:
-        realm = Realm(domain=domain, name=name,
+        realm = Realm(domain=domain, name=name, subdomain=subdomain,
                       restricted_to_domain=restricted_to_domain)
         realm.save()