Add endpoint for generating an image of a letter

The PDF preview is all good, but it’s hard, finickeity and feels dirty
to embed a PDF in a web page. It’s a more natural thing to embed an
image in a web page.

So this commit adds another endpoint to return an image of a letter
template. It generates this image from the PDF preview, so the stack
looks like:

1. `template.png` (generated in admin)
2. `template.pdf` (generated in admin)
3. HTML preview (generated by a `Renderer` in utils)
4. `Template` instance
5. serialised template from API
6. Template stored in database

The library used to convert the PDF to an image is Wand[1], which binds
to ImageMagick underneath. So in order to get this working locally on a
Mac you will probably need to do:
`brew install imagemagick ghostscript cairo pango`.

To get it working on Ubuntu/EC2 is an exercise left to the reader…

1. http://docs.wand-py.org/en/0.4.4/

tests/app/main/views/test_templates.py

@@ -44,6 +44,7 @@ def test_should_show_page_for_one_template(
     'view, expected_content_type',
     [
         ('.view_letter_template_as_pdf', 'application/pdf'),
+        ('.view_letter_template_as_image', 'image/png'),
     ]
 )
 @patch("app.main.views.templates.LetterPreview.__call__")