use dates rather than datetimes when comparing with bst_date

bst_date is a date field. Comparing dates with datetimes in postgres
gets confusing and dangerous. See this example, where a date evaluates
as older than midnight that same day.

```
notification_api=# select '2019-04-01' >= '2019-04-01 00:00';
 ?column?
----------
 f
(1 row)
```

By only using dates everywhere, we reduce the chance of these bugs
happening

tests/app/dao/test_date_utils.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, date
 
 import pytest
 
@@ -40,7 +40,8 @@ def test_get_month_start_and_end_date_in_utc(month, year, expected_start, expect
     (datetime(2018, 3, 31, 23, 0, 0), 2018),
     (datetime(2019, 3, 31, 22, 59, 59), 2018),
     (datetime(2019, 3, 31, 23, 0, 0), 2019),
-    (datetime(2020, 3, 31, 22, 59, 0), 2019),
+    (date(2019, 3, 31), 2018),
+    (date(2019, 4, 1), 2019),
 ])
 def test_get_financial_year_for_datetime(dt, fy):
     assert get_financial_year_for_datetime(dt) == fy