meta tags: Use open graph titles/descriptions.

Apparently, we were incorrectly using constants for title/description
rather than the nice non-constant values from og:title and
og:description in our meta tags.

templates/zerver/base.html

@@ -8,9 +8,13 @@
         {% block title %}
             {% if user_profile and user_profile.realm.name %}
                 <title>{{user_profile.realm.name}} - Zulip</title>
+            {% else %}
+                {% if OPEN_GRAPH_TITLE %}
+                <title>{{ OPEN_GRAPH_TITLE }}</title>
                 {% else %}
                 <title>Zulip</title>
                 {% endif %}
+            {% endif %}
         {% endblock %}
         <link id="favicon" rel="icon" href="/static/images/favicon.svg?v=4">
         <link rel="alternate icon" href="/static/images/favicon.png?v=4">

templates/zerver/meta_tags.html

@@ -1,6 +1,10 @@
 <!-- Google / Search Engine Tags -->
 {% if allow_search_engine_indexing %}
+    {% if OPEN_GRAPH_DESCRIPTION %}
+    <meta name="description" content="{{ OPEN_GRAPH_DESCRIPTION }}">
+    {% else %}
     <meta name="description" content="Zulip combines the immediacy of real-time chat with an email threading model. With Zulip, you can catch up on important conversations while ignoring irrelevant ones.">
+    {% endif %}
 {% else %}
     <meta name="robots" content="noindex,nofollow">
 {% endif %}

zerver/tests/test_docs.py

@@ -75,7 +75,8 @@ class DocPageTest(ZulipTestCase):
             for s in landing_missing_strings:
                 self.assertNotIn(s, str(result.content))
             if not doc_html_str:
-                self.assert_in_success_response(['<meta name="description" content="Zulip combines'], result)
+                # Every page has a meta-description
+                self.assert_in_success_response(['<meta name="description" content="'], result)
             self.assert_not_in_success_response(['<meta name="robots" content="noindex,nofollow">'], result)
 
             # Test the URL on the "zephyr" subdomain with the landing page setting