Clean up code to remove unnecessary paths.

Status code was overcomplex, given how we control the inputs. Now, it
expects a single value, rather than a comma separated list, and if you
give something it doesn't expect it just returns all. Note, it won't
select the correct box - but if you've been manually editing the URL
that's your own problem ¯\_(ツ)_/¯

Also, as this page will only ever be shown from the tour (tutorial),
it doesn't need some non-help things - such as the download csv button
and associated endpoint.

tests/app/main/views/test_notifications.py

@@ -0,0 +1,28 @@
+from freezegun import freeze_time
+import pytest
+from werkzeug.datastructures import MultiDict
+
+from app.main.views.notifications import get_status_arg
+from app.utils import (
+    REQUESTED_STATUSES,
+    FAILURE_STATUSES,
+    SENDING_STATUSES,
+    DELIVERED_STATUSES,
+)
+
+
+@pytest.mark.parametrize('multidict_args, expected_statuses', [
+    ([], REQUESTED_STATUSES),
+    ([('status', '')], REQUESTED_STATUSES),
+    ([('status', 'garbage')], REQUESTED_STATUSES),
+    ([('status', 'sending')], SENDING_STATUSES),
+    ([('status', 'delivered')], DELIVERED_STATUSES),
+    ([('status', 'failed')], FAILURE_STATUSES),
+])
+def test_status_filters(mocker, multidict_args, expected_statuses):
+    mocker.patch('app.main.views.notifications.current_app')
+
+    args = MultiDict(multidict_args)
+    args['status'] = get_status_arg(args)
+
+    assert sorted(args['status']) == sorted(expected_statuses)