Add new tasks for sanitising precompiled letters

Added a task, `sanitise-letter`, that will be called from antivirus when
a letter has passed virus scan. This calls a new task in
template-preview which will sanitise the PDF.

A second new task, `process-sanitised-letter`, will be called from the
template preview task and deals with updating the notification and
moving it to the relevant bucket.

tests/app/letters/test_letter_utils.py

@@ -12,6 +12,7 @@ from app.letters.utils import (
     get_letter_pdf_filename,
     get_letter_pdf_and_metadata,
     letter_print_day,
+    move_sanitised_letter_to_test_or_live_pdf_bucket,
     upload_letter_pdf,
     ScanErrorType, move_failed_pdf, get_folder_name
 )
@@ -338,6 +339,52 @@ def test_get_folder_name_returns_empty_string_for_test_letter():
     assert '' == get_folder_name(datetime.utcnow(), dont_use_sending_date=True)
 
 
+@mock_s3
+def test_move_sanitised_letter_to_live_pdf_bucket(notify_api, mocker):
+    filename = 'my_letter.pdf'
+    source_bucket_name = current_app.config['LETTER_SANITISE_BUCKET_NAME']
+    target_bucket_name = current_app.config['LETTERS_PDF_BUCKET_NAME']
+
+    conn = boto3.resource('s3', region_name='eu-west-1')
+    source_bucket = conn.create_bucket(Bucket=source_bucket_name)
+    target_bucket = conn.create_bucket(Bucket=target_bucket_name)
+
+    s3 = boto3.client('s3', region_name='eu-west-1')
+    s3.put_object(Bucket=source_bucket_name, Key=filename, Body=b'pdf_content')
+
+    move_sanitised_letter_to_test_or_live_pdf_bucket(
+        filename=filename,
+        is_test_letter=False,
+        created_at=datetime.utcnow()
+    )
+
+    assert not [x for x in source_bucket.objects.all()]
+    assert len([x for x in target_bucket.objects.all()]) == 1
+
+
+@mock_s3
+def test_move_sanitised_letter_to_test_pdf_bucket(notify_api, mocker):
+    filename = 'my_letter.pdf'
+    source_bucket_name = current_app.config['LETTER_SANITISE_BUCKET_NAME']
+    target_bucket_name = current_app.config['TEST_LETTERS_BUCKET_NAME']
+
+    conn = boto3.resource('s3', region_name='eu-west-1')
+    source_bucket = conn.create_bucket(Bucket=source_bucket_name)
+    target_bucket = conn.create_bucket(Bucket=target_bucket_name)
+
+    s3 = boto3.client('s3', region_name='eu-west-1')
+    s3.put_object(Bucket=source_bucket_name, Key=filename, Body=b'pdf_content')
+
+    move_sanitised_letter_to_test_or_live_pdf_bucket(
+        filename=filename,
+        is_test_letter=True,
+        created_at=datetime.utcnow()
+    )
+
+    assert not [x for x in source_bucket.objects.all()]
+    assert len([x for x in target_bucket.objects.all()]) == 1
+
+
 @freeze_time('2017-07-07 20:00:00')
 def test_letter_print_day_returns_today_if_letter_was_printed_after_1730_yesterday():
     created_at = datetime(2017, 7, 6, 17, 30)