gallery: Use new response.set_payload method

The `set_payload` method of the `VariedResponse` class takes care of
rendering the Jinja template  or serializing the context object as JSON,
so that no longer needs to be handled by controllers.

src/dcow/gallery.py

@@ -1,4 +1,3 @@
-import jinja2
 import milla.controllers
 import os
 
@@ -13,19 +12,6 @@ DEFAULT_EXTENSIONS = [
     '.tif',
 ]
 
-TMPL_LOADER = jinja2.PackageLoader(__name__.rsplit('.', 1)[0])
-
-
-def render(request, template, context=None):
-    if context is None:
-        context = {}
-    env = jinja2.Environment(loader=TMPL_LOADER)
-    env.globals.update(
-        url=request.create_href,
-        static=request.static_resource,
-    )
-    return env.get_template(template).render(**context)
-
 
 def get_images(path, extensions=None):
     if extensions is None:
@@ -44,7 +30,7 @@ class GalleryController(milla.controllers.HTTPVerbController):
     def GET(self, request):
         response = request.ResponseClass()
         images = get_images(request.config['gallery.screenshot_dir'])
-        response.text = render(request, 'gallery.html.j2', {
-            'images': (i for m, i in sorted(images)),
+        response.set_payload('gallery.html.j2', {
+            'images': [i for m, i in sorted(images)],
         })
         return response