Add rudimentary Jinja2 template tag support

The following tags and filters are supported:

* `wagtailcore`:
  * `pageurl()`
  * `slugurl()`
  * `wagtail_version()`
  * `|pageurl`
* `wagtailimages`:
  * `image()`

Django template tags have been translated to Jinja functions, rather
than custom tags. Functions are easier to use compared to template tags,
and can be composed and combined for greater flexibility.

The template tag libraries have been grouped as Jinja Extensions, which
are loadable via the `extensions` option. An example Django Jinja2
configuration is:

```python
TEMPLATES = [
    # ...
    {
        'BACKEND': 'django.template.backends.jinja2.Jinja2',
        'APP_DIRS': True,
        'OPTIONS': {
            'extensions': [
                'wagtail.wagtailcore.templatetags.jinja2.core',
                'wagtail.wagtailimages.templatetags.jinja2.images',
            ],
        },
    },
]
```

wagtail/wagtailcore/templatetags/jinja2.py

@@ -0,0 +1,22 @@
+import jinja2
+from jinja2.ext import Extension
+
+from .wagtailcore_tags import pageurl, richtext, slugurl, wagtail_version
+
+
+class WagtailCoreExtension(Extension):
+    def __init__(self, environment):
+        super(WagtailCoreExtension, self).__init__(environment)
+
+        self.environment.globals.update({
+            'pageurl': jinja2.contextfunction(pageurl),
+            'slugurl': jinja2.contextfunction(slugurl),
+            'wagtail_version': wagtail_version,
+        })
+        self.environment.filters.update({
+            'richtext': richtext,
+        })
+
+
+# Nicer import names
+core = WagtailCoreExtension

wagtail/wagtailimages/models.py

@@ -448,6 +448,9 @@ class AbstractRendition(models.Model):
         else:
             return mark_safe('<img %s>' % self.attrs)
 
+    def __html__(self):
+        return self.img_tag()
+
     class Meta:
         abstract = True
 

wagtail/wagtailimages/templatetags/jinja2.py

@@ -0,0 +1,39 @@
+from jinja2.ext import Extension
+
+from wagtail.wagtailimages.models import SourceImageIOError
+
+
+def image(image, filterspec, **attrs):
+    if not image:
+        return ''
+
+    try:
+        rendition = image.get_rendition(filterspec)
+    except SourceImageIOError:
+        # It's fairly routine for people to pull down remote databases to their
+        # local dev versions without retrieving the corresponding image files.
+        # In such a case, we would get a SourceImageIOError at the point where we try to
+        # create the resized version of a non-existent image. Since this is a
+        # bit catastrophic for a missing image, we'll substitute a dummy
+        # Rendition object so that we just output a broken link instead.
+        Rendition = image.renditions.model  # pick up any custom Image / Rendition classes that may be in use
+        rendition = Rendition(image=image, width=0, height=0)
+        rendition.file.name = 'not-found'
+
+    if attrs:
+        return rendition.img_tag(attrs)
+    else:
+        return rendition
+
+
+class WagtailImagesExtension(Extension):
+    def __init__(self, environment):
+        super(WagtailImagesExtension, self).__init__(environment)
+
+        self.environment.globals.update({
+            'image': image,
+        })
+
+
+# Nicer import names
+images = WagtailImagesExtension