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.

zproject/jinja2/__init__.py

@@ -0,0 +1,33 @@
+from __future__ import absolute_import  # Python 2 only
+
+from django.contrib.staticfiles.storage import staticfiles_storage
+from django.template.defaultfilters import slugify, pluralize
+from django.core.urlresolvers import reverse
+from django.template.loader import render_to_string
+from django.utils import translation
+from django.http import HttpResponse
+from jinja2 import Environment
+
+from .compressors import compressed_css, minified_js
+
+
+def render_to_response(*args, **kwargs):
+    response = render_to_string(*args, **kwargs)
+    return HttpResponse(response)
+
+
+def environment(**options):
+    env = Environment(**options)
+    env.globals.update({
+        'static': staticfiles_storage.url,
+        'url': reverse,
+        'compressed_css': compressed_css,
+        'minified_js': minified_js,
+    })
+
+    env.install_gettext_translations(translation)
+
+    env.filters['slugify'] = slugify
+    env.filters['pluralize'] = pluralize
+
+    return env