Make text message price dependent on date

So we don’t have to deploy a change on a Saturday.

In the future this could pull from the rates in the database, but while
that code is shifting around I didn’t want to touoch it. We’d also have
to think about caching so as not to have a non-authenticated route
hitting the database.

app/main/views/pricing.py

@@ -1,19 +1,28 @@
+from datetime import datetime
+
 from flask import current_app, render_template
 from flask_login import current_user
 from notifications_utils.international_billing_rates import (
     INTERNATIONAL_BILLING_RATES,
 )
+from notifications_utils.timezones import convert_utc_to_bst
 
 from app.main import main
 from app.main.forms import SearchByNameForm
 from app.main.views.sub_navigation_dictionaries import pricing_nav
 
 
+def get_current_sms_rate():
+    if convert_utc_to_bst(datetime.utcnow()) < convert_utc_to_bst(datetime(2022, 5, 1)):
+        return '1.61'
+    return '1.72'
+
+
 @main.route('/pricing')
 def pricing():
     return render_template(
         'views/pricing/index.html',
-        sms_rate=0.0158,
+        sms_rate=get_current_sms_rate(),
         international_sms_rates=sorted([
             (cc, country['names'], country['billable_units'])
             for cc, country in INTERNATIONAL_BILLING_RATES.items()