remove pdf/png code from admin app entirely

while PDFs work on paas, they only do that because it turns out the
python buildpack happens to have imagemagick preinstalled - if that
ever changes then it'd break. so move those to the template preview
service. This also means we can get rid of weazyprint and wand
dependencies

app/template_previews.py

@@ -1,17 +1,29 @@
-from flask import current_app, current_service
+from flask import current_app
 import requests
 
+from app import current_service
 
-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())
+
+class TemplatePreview:
+    @classmethod
+    def from_database_object(cls, template, filetype, values=None):
+        data = {
+            "letter_contact_block": current_service['letter_contact_block'],
+            "admin_base_url": current_app.config['ADMIN_BASE_URL'],
+            "template": template,
+            "values": values
+        }
+        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())
+
+    @classmethod
+    def from_utils_template(cls, template, filetype):
+        return cls.from_database_object(
+            template._template,
+            filetype,
+            template.values
+        )