Refactor PNG generating endpoint for code reuse

The way we generate PNGs is the same for all the different endpoints,
so the code to do this should be shared.

app/utils.py

@@ -1,6 +1,6 @@
 import re
 import csv
-from io import StringIO
+from io import StringIO, BytesIO
 from os import path
 from functools import wraps
 import unicodedata
@@ -8,6 +8,8 @@ import unicodedata
 from flask import (abort, current_app, session, request, redirect, url_for)
 from flask_login import current_user
 
+from wand.image import Image
+
 from notifications_utils.template import (
     SMSPreviewTemplate,
     EmailPreviewTemplate,
@@ -257,3 +259,17 @@ def get_template(
             return LetterPreviewTemplate(
                 template
             )
+
+
+def png_from_pdf(pdf_endpoint):
+    output = BytesIO()
+    with Image(
+        blob=pdf_endpoint.get_data()
+    ) as image:
+        with image.convert('png') as converted:
+            converted.save(file=output)
+    output.seek(0)
+    return dict(
+        filename_or_fp=output,
+        mimetype='image/png',
+    )