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]
hackerkid