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/

app/assets/stylesheets/components/letter.scss

@@ -1,6 +1,7 @@
 $outline-width: 5px;
 
 .letter {
+
   font-family: Helvetica, Arial, sans-serif;
   box-shadow:
   1px 1px 0 0 $panel-colour,
@@ -8,6 +9,16 @@ $outline-width: 5px;
   -1px 1px 0 0 $panel-colour,
   -2px 2px 0 0 rgba($panel-colour, 0.5);
   outline: $outline-width solid rgba($text-colour, 0.1);
-  padding: 20px;
+  padding: 0;
   margin: $outline-width $outline-width $gutter;
+
+  a {
+    display: block;
+  }
+
+  img {
+    display: block;
+    max-width: 100%;
+  }
+
 }

app/main/views/templates.py

@@ -1,10 +1,12 @@
 from datetime import datetime, timedelta
+from io import BytesIO
 from string import ascii_uppercase
 
-from flask import request, render_template, redirect, url_for, flash, abort
+from flask import request, render_template, redirect, url_for, flash, abort, send_file
 from flask_login import login_required
 from flask_weasyprint import HTML, render_pdf
 from dateutil.parser import parse
+from wand.image import Image
 
 from notifications_utils.template import Template
 from notifications_utils.recipients import first_column_headings
@@ -64,6 +66,20 @@ def view_letter_template_as_pdf(service_id, template_id):
     return render_pdf(HTML(string=template.rendered))
 
 
+@main.route("/services/<service_id>/templates/<template_id>.png")
+@login_required
+@user_has_permissions('view_activity', admin_override=True)
+def view_letter_template_as_image(service_id, template_id):
+    output = BytesIO()
+    with Image(
+        blob=view_letter_template_as_pdf(service_id, template_id).get_data()
+    ) as image:
+        with image.convert('png') as converted:
+            converted.save(file=output)
+    output.seek(0)
+    return send_file(output, mimetype='image/png')
+
+
 @main.route("/services/<service_id>/templates/<template_id>/version/<int:version>")
 @login_required
 @user_has_permissions(