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

tests/app/dao/test_permissions_dao.py

@@ -4,13 +4,12 @@ from tests.app.db import create_service
 
 def test_get_permissions_by_user_id_returns_all_permissions(sample_service):
     permissions = permission_dao.get_permissions_by_user_id(user_id=sample_service.users[0].id)
-    assert len(permissions) == 8
+    assert len(permissions) == 7
     assert sorted(["manage_users",
                    "manage_templates",
                    "manage_settings",
                    "send_texts",
                    "send_emails",
-                   "send_letters",
                    "manage_api_keys",
                    "view_activity"]) == sorted([i.permission for i in permissions])
 
@@ -20,6 +19,6 @@ def test_get_permissions_by_user_id_returns_only_active_service(sample_user):
     inactive_service = create_service(user=sample_user, service_name="Inactive service", active=False)
 
     permissions = permission_dao.get_permissions_by_user_id(user_id=sample_user.id)
-    assert len(permissions) == 8
+    assert len(permissions) == 7
     assert active_service in [i.service for i in permissions]
     assert inactive_service not in [i.service for i in permissions]