Insert initial letter rates.

Create letter rates dao.
Query to fetch letter rates.

app/dao/letter_rate_dao.py

@@ -0,0 +1,23 @@
+from app import db
+from app.dao.dao_utils import transactional
+from app.models import LetterRate
+
+
+@transactional
+def dao_create_letter_rate(letter_rate):
+    db.session.add(letter_rate)
+
+
+def get_letter_rates_for_daterange(date, crown, sheet_count, post_class='second'):
+    rates = LetterRate.query.filter(
+        LetterRate.start_date <= date
+    ).filter((LetterRate.end_date == None) |  # noqa
+             (LetterRate.end_date > date)
+             ).filter(
+        LetterRate.crown == crown
+    ).filter(
+        LetterRate.sheet_count == sheet_count
+    ).filter(
+        LetterRate.post_class == post_class
+    ).all()
+    return rates