settings: Set REALMS_HAVE_SUBDOMAINS by default.

This change means that almost every Zulip server out there will now be
using subdomains for every realm.  There are a few complications noted
in the release notes.

docs/changelog.md

@@ -44,6 +44,14 @@ recommend
 [running these migrations manually](expensive-migrations.html) before
 starting the upgrade.
 
+* We fixed Zulip's previously buggy support for multiple organizations
+on the same Zulip server.  A consequence of doing this correctly means
+is that we require each organization to have its own subdomain.  This
+change should have no effect for the vast major of Zulip servers that
+only have one organization.  If you manage a server that hosts
+multiple organizations, you'll want to read
+[our guide on multiple organizations](prod-multiple-organizations.html).
+
 Full feature Changelog:
 
 - Simplified the process for installing a new Zulip server.

docs/prod-maintain-secure-upgrade.md

@@ -453,3 +453,7 @@ server, and suggested procedure.
 There are a large number of useful management commands under
 `zerver/management/commands/`; you can also see them listed using
 `./manage.py` with no arguments.
+
+## Hosting multiple Zulip organizations
+
+This is explained in detail on [its own page](prod-multiple-organizations.html).

docs/prod-multiple-organizations.md

@@ -0,0 +1,49 @@
+# Hosting multiple organizations
+
+The vast majority of Zulip servers host just a single organization.
+This article documents what's involved in hosting multiple Zulip
+organizations on a single server.
+
+Throughout this article, we'll assume you're working on a zulip server
+with hostname `zulip.example.com`.
+
+## Subdomains
+
+Zulip's approach for supporting multiple organizations on a single
+Zulip server is for each organization to be hosted on its own
+subdomain.  E.g. you'd have `org1.zulip.example.com` and
+`org2.zulip.example.com`.
+
+Web security standards mean that one subdomain per organization is
+required to support a use logging into multiple organizations on a
+server at the same time.
+
+When you want to create a new organization, you need to do a few
+things:
+
+* If you're using Zulip older than 1.7, you'll need to set
+  `REALMS_HAVE_SUBDOMAINS=True` in your `/etc/zulip/settings.py`
+  file.  That setting is the default in 1.7 and later.
+* Make sure you have SSL certificates for all of the subdomains you're
+  going to use.  If you're using
+  [our LetsEncrypt instructions](ssl-certificates.html), it's easy to
+  just specify multiple subdomains in your certificate request.
+* If necessary, modify your `nginx` configuration to use your new
+  certificates.
+* Use `./manage.py generate_realm_creation_link` again to create your
+  new organization.  Review
+  [the install instructions](prod-install.html) if you need a
+  refresher on how this works.
+
+For servers hosting a large number of organizations, like
+[zulipchat.com](https://zulipchat.com), one can set
+`ROOT_DOMAIN_LANDING_PAGE = True` in `/etc/zulip/settings.py` so that
+the homepage for the server is a copy of the Zulip homepage.
+
+### The root domain
+
+Most Zulip servers host a single Zulip organization on the root domain
+(i.e. `zulip.example.com`).  The way this is implemented internally
+involves the organization having the empty string (`''`) as its
+"subdomain".  You can mix having an organization on the root domain
+and some others on subdomains (e.g. `it.zulip.example.com`).

zproject/settings.py

@@ -181,7 +181,7 @@ DEFAULT_SETTINGS = {'TWITTER_CONSUMER_KEY': '',
                     'VERBOSE_SUPPORT_OFFERS': False,
                     'STATSD_HOST': '',
                     'OPEN_REALM_CREATION': False,
-                    'REALMS_HAVE_SUBDOMAINS': False,
+                    'REALMS_HAVE_SUBDOMAINS': True,
                     'ROOT_DOMAIN_LANDING_PAGE': False,
                     'ROOT_SUBDOMAIN_ALIASES': ["www"],
                     'REMOTE_POSTGRES_HOST': '',