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]
As they stand now, alert words tests will cause a race condition with
all subsequent tests which access the UserProfile object these tests
modify. Currently, if we modify alert words, we don't get any
notification from the server, issue reported at #1269. Consequently, we
can't wait on any condition to avoid the race condition. The best option
is to wait for the fix of #1269 and modify the tests in that issue.
Fixes: #1244
