Add a document download client

Allows uploading documents to the Document Download API.
The client is configured with an API host and auth token. There's
no need for a flag to disable the client in the test environments
at the moment since the upload is only triggered by a specific
payload which would only be sent with an explicit goal of using
document download.

app/clients/document_download.py

@@ -0,0 +1,27 @@
+import base64
+
+import requests
+
+
+class DocumentDownloadClient:
+
+    def init_app(self, app):
+        self.api_host = app.config['DOCUMENT_DOWNLOAD_API_HOST']
+        self.auth_token = app.config['DOCUMENT_DOWNLOAD_API_KEY']
+
+    def get_upload_url(self, service_id):
+        return "{}/services/{}/documents".format(self.api_host, service_id)
+
+    def upload_document(self, service_id, file_contents):
+        response = requests.post(
+            self.get_upload_url(service_id),
+            headers={
+                'Authorization': "Bearer {}".format(self.auth_token),
+            },
+            files={
+                'document': base64.b64decode(file_contents)
+            }
+        )
+        response.raise_for_status()
+
+        return response.json()['document']['url']