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.

app/cronitor.py

@@ -5,12 +5,15 @@ from flask import current_app
 
 
 def cronitor(task_name):
-    # check if task_name is in config
     def decorator(func):
         def ping_cronitor(command):
             if not current_app.config['CRONITOR_ENABLED']:
                 return
 
+            # it's useful to have a log that a periodic task has started in case it
+            # get stuck without generating any other logs - we know it got this far
+            current_app.logger.info(f'Pinging Cronitor for Celery task {task_name}')
+
             task_slug = current_app.config['CRONITOR_KEYS'].get(task_name)
             if not task_slug:
                 current_app.logger.error(