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.

zerver/lib/upload.py

@@ -3,7 +3,7 @@ from __future__ import absolute_import
 from django.conf import settings
 from django.template.defaultfilters import slugify
 from django.utils.encoding import force_text
-from django.utils.safestring import mark_safe
+from jinja2 import Markup as mark_safe
 import unicodedata
 
 from zerver.lib.avatar import user_avatar_hash