Generate letter previews in the new notifications-template-preview service

by calling through to this, we no longer require admin app to ship
with binary dependencies (imagemagick/cairo/etc)

app/template_previews.py

@@ -0,0 +1,17 @@
+from flask import current_app, current_service
+import requests
+
+
+def get_template_preview(template, filetype):
+    data = {
+        "letter_contact_block": current_service['letter_contact_block'],
+        "admin_base_url": current_app.config['ADMIN_BASE_URL'],
+        "template": template,
+        "values": None
+    }
+    resp = requests.post(
+        '{}/preview.{}'.format(current_app.config['TEMPLATE_PREVIEW_SERVICE_URL'], filetype),
+        json=data,
+        headers={'Authorization': 'Token my-secret-key'}
+    )
+    return (resp.content, resp.status_code, resp.headers.items())