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/confirmation/confirm_email_change.html

@@ -1,5 +1,9 @@
 {% extends "zerver/portico.html" %}
 
+{% block title %}
+<title>{{ _("Email changed") }} | Zulip</title>
+{% endblock %}
+
 {% block portico_content %}
 
 <div class="app confirm-email-change-page flex full-page">

templates/confirmation/confirm_preregistrationuser.html

@@ -1,6 +1,10 @@
 {% extends "zerver/base.html" %}
 {% set entrypoint = "confirm-preregistrationuser" %}
 
+{% block title %}
+<title>{{ _("Confirming your email address") }} | Zulip</title>
+{% endblock %}
+
 {% block content %}
 
 {#

templates/confirmation/link_does_not_exist.html

@@ -1,5 +1,9 @@
 {% extends "zerver/portico.html" %}
 
+{% block title %}
+<title>{{ _("Confirmation link does not exist") }} | Zulip</title>
+{% endblock %}
+
 {% block portico_content %}
 <div class="error_page">
     <div class="container">

templates/confirmation/link_expired.html

@@ -1,5 +1,9 @@
 {% extends "zerver/portico.html" %}
 
+{% block title %}
+<title>{{ _("Confirmation link expired or deactivated") }} | Zulip</title>
+{% endblock %}
+
 {% block portico_content %}
 <div class="error_page">
     <div class="container">

templates/confirmation/link_malformed.html

@@ -1,5 +1,9 @@
 {% extends "zerver/portico.html" %}
 
+{% block title %}
+<title>{{ _("Confirmation link malformed") }} | Zulip</title>
+{% endblock %}
+
 {% block portico_content %}
 <div class="error_page">
     <div class="container">