templates: Add HTML title element to templates that extend base/portico.

Adds HTML title elements to templates that extend either `base.html`,
`portico.html` or `portico_signup.html`, and that are not website
portico landing pages that will use the `PAGE_TITLE` variable to set
the HTML title element (see following commit in series).

Also, updates some templates for missing translation tags.

As a general rule, we want the title element (and page content)
translated. Exceptions that are updated in this commit are templates
used in the development environment, analytics templates that are used
by staff and templates related to Zephyr.

templates/500.html

@@ -1,5 +1,9 @@
 {% extends "zerver/portico.html" %}
 
+{% block title %}
+<title>{{ _("Internal server error") }} | Zulip</title>
+{% endblock %}
+
 {% block customhead %}
 {{ super() }}
 <meta http-equiv="refresh" content="60;URL='/'" />
@@ -14,10 +18,12 @@
             <img src="/static/images/errors/500art.svg" alt=""/>
             <div class="errorbox">
                 <div class="errorcontent">
-                    <h1 class="lead">Internal server error</h1>
+                    <h1 class="lead">{{ _("Internal server error") }}</h1>
+                    {% trans %}
                     <p>This Zulip server is currently experiencing some technical difficulties. Sorry about that!</p>
                     <p>The page will reload automatically soon after service is restored.</p>
                     <p>If you'd like, you can <a href="mailto:{{ support_email }}">drop us a line</a> to let us know what happened.</p>
+                    {% endtrans %}
                 </div>
 
             </div>