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]

zproject/dev_settings.py

@@ -4,8 +4,8 @@
 from .prod_settings_template import *
 
 LOCAL_UPLOADS_DIR = 'var/uploads'
-EXTERNAL_HOST = 'localhost:9991'
-ALLOWED_HOSTS = ['localhost']
+EXTERNAL_HOST = 'zulipdev.com:9991'
+ALLOWED_HOSTS = ['*']
 AUTHENTICATION_BACKENDS = ('zproject.backends.DevAuthBackend',)
 # Add some of the below if you're testing other backends
 # AUTHENTICATION_BACKENDS = ('zproject.backends.EmailAuthBackend',
@@ -20,6 +20,9 @@ EXTRA_INSTALLED_APPS = ["zilencer", "analytics"]
 # Disable Camo in development
 CAMO_URI = ''
 OPEN_REALM_CREATION = True
+# Default to subdomains disabled in development until we can update
+# the development documentation to make sense with subdomains.
+REALMS_HAVE_SUBDOMAINS = False
 TERMS_OF_SERVICE = 'zproject/terms.md.template'
 
 SAVE_FRONTEND_STACKTRACES = True