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/0341_new_letter_rates.py

@@ -12,8 +12,6 @@ from datetime import datetime
 from alembic import op
 from sqlalchemy.sql import text
 
-from app.models import LetterRate
-
 
 revision = '0341_new_letter_rates'
 down_revision = '0340_stub_training_broadcasts'
@@ -36,24 +34,24 @@ def get_new_rate(sheet_count, post_class):
 
 def upgrade():
     conn = op.get_bind()
-    conn.execute(text("UPDATE letter_rates SET end_date = :start WHERE end_date IS NULL"), start=CHANGEOVER_DATE)
+    # conn.execute(text("UPDATE letter_rates SET end_date = :start WHERE end_date IS NULL"), start=CHANGEOVER_DATE)
 
-    op.bulk_insert(LetterRate.__table__, [
-        {
-            'id': uuid.uuid4(),
-            'start_date': CHANGEOVER_DATE,
-            'end_date': None,
-            'sheet_count': sheet_count,
-            'rate': get_new_rate(sheet_count, post_class),
-            'crown': crown,
-            'post_class': post_class,
-        }
-        for sheet_count, crown, post_class in itertools.product(
-            range(1, 6),
-            [True, False],
-            ['first', 'second', 'europe', 'rest-of-world']
-        )
-    ])
+    # op.bulk_insert('letter_rates', [
+    #     {
+    #         'id': uuid.uuid4(),
+    #         'start_date': CHANGEOVER_DATE,
+    #         'end_date': None,
+    #         'sheet_count': sheet_count,
+    #         'rate': get_new_rate(sheet_count, post_class),
+    #         'crown': crown,
+    #         'post_class': post_class,
+    #     }
+    #     for sheet_count, crown, post_class in itertools.product(
+    #         range(1, 6),
+    #         [True, False],
+    #         ['first', 'second', 'europe', 'rest-of-world']
+    #     )
+    # ])
 
 def downgrade():
     # Make sure you've thought about billing implications etc before downgrading!