Add log of notifications to API integration page

Now that we’ve removed simulated notifications from the dashboard and
activity pages they’re not visible anywhere in the app.

While they should’t be visible to non-technical users, developers have
a real need for Notify to confirm that their code is doing what they
expect. This is needed especially when they’re just getting started with
Notify.

There’s no way of seeing this info from the API either, because a key
can only get notifications created with a key of that type.

It doesn’t make sense to make this a ‘mode’ of the dashboard or activity
because the information about notifications that developers need is
also different. So this commit adds up to 50 of the most recent
notifications sent via the API to the page that developers use as their
‘home’ page.

This also lets us explain the 7 days thing to developers via the
empty slate state of this area of the page.

app/__init__.py

@@ -1,7 +1,9 @@
+import ago
 import os
 import re
 import urllib
 import dateutil
+from datetime import datetime, timedelta, timezone
 import pytz
 from flask import (
     Flask,
@@ -115,6 +117,7 @@ def create_app():
     application.add_template_filter(format_date)
     application.add_template_filter(format_date_normal)
     application.add_template_filter(format_date_short)
+    application.add_template_filter(format_delta)
     application.add_template_filter(format_notification_status)
     application.add_template_filter(format_notification_status_as_time)
     application.add_template_filter(format_notification_status_as_field_status)
@@ -251,6 +254,19 @@ def format_date_short(date):
     return gmt_timezones(date).strftime('%d %B').lstrip('0')
 
 
+def format_delta(date):
+    return ago.human(
+        (
+            datetime.now(timezone.utc)
+        ) - (
+            dateutil.parser.parse(date)
+        ),
+        future_tense='{} from now',  # No-one should ever see this
+        past_tense='{} ago',
+        precision=1
+    )
+
+
 def valid_phone_number(phone_number):
     try:
         validate_phone_number(phone_number)