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

app/dao/date_util.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, date, time
 
 from notifications_utils.timezones import convert_bst_to_utc
 import pytz
@@ -64,6 +64,9 @@ def get_current_financial_year_start_year():
 
 
 def get_financial_year_for_datetime(start_date):
+    if type(start_date) == date:
+        start_date = datetime.combine(start_date, time.min)
+
     year = int(start_date.strftime('%Y'))
     if start_date < get_april_fools(year):
         return year - 1

app/dao/fact_billing_dao.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta, time
+from datetime import datetime, timedelta, time, date
 
 from flask import current_app
 from notifications_utils.timezones import convert_bst_to_utc, convert_utc_to_bst
@@ -7,7 +7,6 @@ from sqlalchemy import func, case, desc, Date, Integer, and_
 
 from app import db
 from app.dao.date_util import (
-    get_april_fools as financial_year_start,
     get_financial_year,
     get_financial_year_for_datetime
 )
@@ -34,7 +33,7 @@ from app.utils import get_london_midnight_in_utc
 def fetch_sms_free_allowance_remainder(start_date):
     # ASSUMPTION: AnnualBilling has been populated for year.
     billing_year = get_financial_year_for_datetime(start_date)
-    start_of_year = convert_utc_to_bst(financial_year_start(billing_year))
+    start_of_year = date(billing_year, 4, 1)
 
     billable_units = func.coalesce(func.sum(FactBilling.billable_units * FactBilling.rate_multiplier), 0)
 

app/platform_stats/rest.py

@@ -50,15 +50,15 @@ def validate_date_range_is_within_a_financial_year(start_date, end_date):
     if start_fy != end_fy:
         raise InvalidRequest(message="Date must be in a single financial year.", status_code=400)
 
+    return start_date, end_date
+
 
 @platform_stats_blueprint.route('usage-for-all-services')
 def get_usage_for_all_services():
     start_date = request.args.get('start_date')
     end_date = request.args.get('end_date')
 
-    validate_date_range_is_within_a_financial_year(start_date, end_date)
-    start_date = datetime.strptime(start_date, "%Y-%m-%d")
-    end_date = datetime.strptime(end_date, "%Y-%m-%d")
+    start_date, end_date = validate_date_range_is_within_a_financial_year(start_date, end_date)
 
     sms_costs = fetch_sms_billing_for_all_services(start_date, end_date)
     letter_costs = fetch_letter_costs_for_all_services(start_date, end_date)