5xx: Remove hardcoded twitter/email contacts from nginx 500 page.

There's no correct contact that we can list here unconditionally;
previously, we had people emailing zulip-devel@ because their server
was misconfigured.

Addresses zulip#696 enough that it's no longer a priority issue.

static/html/5xx.html

@@ -33,14 +33,8 @@
         <br/>
         <p class="lead">Internal server error.</p>
         <br/>
-
-        <p>Well oops. This one's probably our fault. Sorry about that! Updates on service availability are posted to
-        <a class="twitter-timeline" href="https://twitter.com/ZulipStatus"
-            data-widget-id="443457763394334720"
-            data-screen-name="ZulipStatus"
-            >@ZulipStatus on Twitter</a>.</p>
-
-        <p>If you'd like, you can <a href="mailto:[email protected]?Subject=500%20error%20on%20%7Bwhich%20URL%3F%7D&Body=Hi%20there%21%0A%0AI%20was%20trying%20to%20do%20%7Bwhat%20were%20you%20trying%20to%20do%3F%7D%20at%20around%20%7Bwhen%20was%20this%3F%7D%20when%20I%20got%20a%20500%20error%20while%20accessing%20%7Bwhich%20URL%3F%7D.%0A%0AThanks!%0A%0ASincerely%2C%20%0A%0A%7BYour%20name%7D">drop us a line</a> to let us know what happened.</p>
+        <!-- TODO: Make nginx 5xx error page customizable -->
+        <!-- This is tricky because it's not served by Django -->
     </div>
 </div>
 

templates/500.html

@@ -8,14 +8,9 @@
 {% block portico_content %}
 
 <br/>
-<p class="lead">500: Internal server error.</p>
+<p class="lead">Internal server error.</p>
 
-<p>Well oops. This one's probably our fault. Sorry about that! Updates on
-service availability are posted to
-<a class="twitter-timeline" href="https://twitter.com/ZulipStatus"
-   data-widget-id="443457763394334720"
-   data-screen-name="ZulipStatus"
->@ZulipStatus on Twitter</a>.</p>
+<p>Well oops. This one's probably our fault. Sorry about that!</p>
 
 <p>If you'd like, you can <a href="mailto:{{ zulip_admin }}?Subject=500%20error%20on%20%7Bwhich%20URL%3F%7D&Body=Hi%20there%21%0A%0AI%20was%20trying%20to%20do%20%7Bwhat%20were%20you%20trying%20to%20do%3F%7D%20at%20around%20%7Bwhen%20was%20this%3F%7D%20when%20I%20got%20a%20500%20error%20while%20accessing%20%7Bwhich%20URL%3F%7D.%0A%0AThanks!%0A%0ASincerely%2C%20%0A%0A%7BYour%20name%7D">drop us a line</a> to let us know what happened.</p>