upload letter pdfs with retention tag

so we can delete them automatically with s3's lifecycle policy

app/variables.py

@@ -1,3 +1,10 @@
 # all jobs for letters created via the api must have this filename
 LETTER_API_FILENAME = 'letter submitted via api'
 LETTER_TEST_API_FILENAME = 'test letter submitted via api'
+
+
+# S3 tags
+class Retention:
+    KEY = 'retention'
+
+    ONE_WEEK = 'ONE_WEEK'