Log activity on all periodic Celery tasks

As stated in the comment, this would have been helpful during an
incident to give further reassurance that a task had at least
started running - at the time the only evidence for this was the
Cronitor dashboard itself, which we don't often look at.

I've removed other, equivalent "starting" logs, but kept those
that provide additional information in the log message.

tests/app/test_cronitor.py

@@ -1,3 +1,4 @@
+import logging
 from urllib import parse
 
 import pytest
@@ -75,6 +76,9 @@ def test_cronitor_does_nothing_if_cronitor_not_enabled(notify_api, rmock):
 
 
 def test_cronitor_does_nothing_if_name_not_recognised(notify_api, rmock, caplog):
+    # ignore "INFO" log about task starting
+    caplog.set_level(logging.ERROR)
+
     with set_config_values(notify_api, {
         'CRONITOR_ENABLED': True,
         'CRONITOR_KEYS': {'not-hello': 'other'}