Remove letters-related code (#175)

This deletes a big ol' chunk of code related to letters. It's not everything—there are still a few things that might be tied to sms/email—but it's the the heart of letters function. SMS and email function should be untouched by this.

Areas affected:

- Things obviously about letters
- PDF tasks, used for precompiling letters
- Virus scanning, used for those PDFs
- FTP, used to send letters to the printer
- Postage stuff

migrations/versions/0317_uploads_for_all.py

@@ -6,7 +6,6 @@ Create Date: 2019-05-13 10:44:51.867661
 
 """
 from alembic import op
-from app.models import UPLOAD_LETTERS
 
 
 revision = '0317_uploads_for_all'
@@ -18,7 +17,7 @@ def upgrade():
         INSERT INTO
             service_permissions (service_id, permission, created_at)
         SELECT
-            id, '{permission}', now()
+            id, 'upload_letters', now()
         FROM
             services
         WHERE
@@ -28,14 +27,10 @@ def upgrade():
                     service_permissions
                 WHERE
                     service_id = services.id and
-                    permission = '{permission}'
+                    permission = 'upload_letters'
            )
-    """.format(
-        permission=UPLOAD_LETTERS
-    ))
+    """)
 
 
 def downgrade():
-    op.execute("DELETE from service_permissions where permission = '{}'".format(
-        UPLOAD_LETTERS
-    ))
+    op.execute("DELETE from service_permissions where permission = 'upload_letters'")