Convert Zulip to use Jinja2 templates.

This results in a substantial performance improvement for all of
Zulip's backend templates.

Changes in templates:
- Change `block.super` to `super()`.
- Remove `load` tag because Jinja2 doesn't support it.
- Use `minified_js()|safe` instead of `{% minified_js %}`.
- Use `compressed_css()|safe` instead of `{% compressed_css %}`.
- `forloop.first` -> `loop.first`.
- Use `{{ csrf_input }}` instead of `{% csrf_token %}`.
- Use `{# ... #}` instead of `{% comment %}`.
- Use `url()` instead of `{% url %}`.
- Use `_()` instead of `{% trans %}` because in Jinja `trans` is a block tag.
- Use `{% trans %}` instead of `{% blocktrans %}`.
- Use `{% raw %}` instead of `{% verbatim %}`.

Changes in tools:
- Check for `trans` block in `check-templates` instead of `blocktrans`

Changes in backend:
- Create custom `render_to_response` function which takes `request` objects
  instead of `RequestContext` object. There are two reasons to do this:
    1. `RequestContext` is not compatible with Jinja2
    2. `RequestContext` in `render_to_response` is deprecated.
- Add Jinja2 related support files in zproject/jinja2 directory. It
  includes a custom backend and a template renderer, compressors for js
  and css and Jinja2 environment handler.
- Enable `slugify` and `pluralize` filters in Jinja2 environment.

Fixes #620.

templates/zerver/accounts_accept_terms.html

@@ -1,34 +1,33 @@
 {% extends "zerver/portico_signup.html" %}
-{% load i18n %}
-{% comment %}
+{#
 Allow the user to accept the terms, creating an email record of that fact.
-{% endcomment %}
+#}
 
 {% block for_you %}for {% if company_name %} {{company_name}} {% else %} __________ {% endif %} {% endblock %}
 {% block portico_content %}
 
-<p>({% trans "Welcome! We think you'll like it here" %}.)</p>
+<p>({{ _("Welcome! We think you'll like it here") }}.)</p>
 
 
 <div class="pitch">
     <hr/>
-    <p>{% trans "You're almost there. We just need you to do one last thing" %}.</p>
-    <h3>{% trans "Accept the Zulip terms of service" %}</h3>
+    <p>{{ _("You're almost there. We just need you to do one last thing") }}.</p>
+    <h3>{{ _("Accept the Zulip terms of service") }}</h3>
 </div>
 
-<form method="post" class="form-horizontal" id="registration" action="{% url 'zerver.views.accounts_accept_terms' %}">
-{% csrf_token %}
+<form method="post" class="form-horizontal" id="registration" action="{{ url('zerver.views.accounts_accept_terms') }}">
+{{ csrf_input }}
     <div class="control-group">
-        <label for="id_email" class="control-label">{% trans "Email" %}</label>
+        <label for="id_email" class="control-label">{{ _("Email") }}</label>
         <div class="controls fakecontrol">
             <p>{{ email }}</p>
         </div>
     </div>
     <div class="control-group">
-        <label for="id_full_name" class="control-label">{% trans "Your name" %}</label>
+        <label for="id_full_name" class="control-label">{{ _("Your name") }}</label>
         <div class="controls">
             <input id="id_full_name" class="required" type="text" name="full_name"
-                   value="{% if form.full_name.value %}{{ form.full_name.value }}{% endif %}"
+                   value="{% if form.full_name.value() %}{{ form.full_name.value() }}{% endif %}"
                    maxlength="100" />
             {% if form.full_name.errors %}
                 {% for error in form.full_name.errors %}
@@ -41,17 +40,17 @@ Allow the user to accept the terms, creating an email record of that fact.
     <div class="control-group">
         <div class="controls">
             <label class="checkbox">
-                {% comment %}
+                {#
                 This is somewhat subtle.
                 Checkboxes have a name and value, and when the checkbox is ticked, the form posts
                 with name=value. If the checkbox is unticked, the field just isn't present at all.
 
                 This is distinct from 'checked', which determines whether the checkbox appears
                 at all. (So, it's not symmetric to the code above.)
-                {% endcomment %}
+                #}
                 <input id="id_terms" class="required" type="checkbox" name="terms"
-                       {% if form.terms.value %}checked="checked"{% endif %} />
-                {% trans "I agree to the" %} <a href="/terms">{% trans "Terms of Service" %}</a>.
+                       {% if form.terms.value() %}checked="checked"{% endif %} />
+                {{ _("I agree to the") }} <a href="/terms">{{ _("Terms of Service") }}</a>.
             </label>
             {% if form.terms.errors %}
                 {% for error in form.terms.errors %}