hello: Add link and label for latest Zulip release.

Tweaked by tabbott to add the plumbing for the 3 variables we need and
add the links, and move to a plain-text styling.

docs/subsystems/release-checklist.md

@@ -33,7 +33,7 @@ preparing a new release.
 
 * Do final updates to `changelog.md`, for any final changes and with
   any revisions from the draft blog post.  (And the date!)
-* Update `ZULIP_VERSION` in `version.py`.
+* Update `ZULIP_VERSION`, `LATEST_RELEASE_VERSION`, etc. in `version.py`.
 * Update `version` and/or `release` in `docs/conf.py` (ReadTheDocs meta tags).
   Leave "+git" off.
 * Use `build-release-tarball` to generate a final release tarball.

templates/zerver/hello.html

@@ -365,6 +365,10 @@
                     <a href="https://github.com/RocketChat/Rocket.Chat/graphs/contributors">Rocket.Chat</a>,
                     and <a href="https://github.com/matrix-org/synapse/graphs/contributors">matrix.org</a>.
                 </p>
+                <p>
+                    <a href="https://zulip.readthedocs.io/en/stable/production/install.html"
+                    class="button">Install Zulip {{ latest_release_version }}</a> or <a href="{{ latest_release_announcement }}">read the Zulip {{ latest_major_version }} release announcement</a>.
+                </p>
             </div>
         </div>
     </div>

version.py

@@ -1,4 +1,7 @@
 ZULIP_VERSION = "1.9.0+git"
+LATEST_MAJOR_VERSION = "1.9"
+LATEST_RELEASE_VERSION = "1.9.0"
+LATEST_RELEASE_ANNOUNCEMENT = "https://blog.zulip.org/2018/11/07/zulip-1-9-released/"
 
 # Bump the minor PROVISION_VERSION to indicate that folks should provision
 # only when going from an old version of the code to a newer version. Bump

zerver/context_processors.py

@@ -20,7 +20,8 @@ from zerver.lib.send_email import FromAddress
 from zerver.lib.subdomains import get_subdomain
 from zerver.lib.realm_icon import get_realm_icon_url
 
-from version import ZULIP_VERSION
+from version import ZULIP_VERSION, LATEST_RELEASE_VERSION, \
+    LATEST_RELEASE_ANNOUNCEMENT, LATEST_MAJOR_VERSION
 
 def common_context(user: UserProfile) -> Dict[str, Any]:
     """Common context used for things like outgoing emails that don't
@@ -136,6 +137,9 @@ def zulip_default_context(request: HttpRequest) -> Dict[str, Any]:
         'jitsi_server_url': settings.JITSI_SERVER_URL,
         'two_factor_authentication_enabled': settings.TWO_FACTOR_AUTHENTICATION_ENABLED,
         'zulip_version': ZULIP_VERSION,
+        'latest_release_version': LATEST_RELEASE_VERSION,
+        'latest_major_version': LATEST_MAJOR_VERSION,
+        'latest_release_announcement': LATEST_RELEASE_ANNOUNCEMENT,
         'user_is_authenticated': user_is_authenticated,
         'settings_path': settings_path,
         'secrets_path': secrets_path,