Fixes: #486

so you can see whether it's up or down at first glance

If the notification does not exist, or is more than a hour
old, return HTTP 200 (instead of 400 or 404) so the other
party doesn't retry over and over again.

Accept all expressions that croniter accepts.
If cron-descriptor throws an exception, don't show the
description to the user.

Fixes: #472

Problem: if you use uBlock Origin, and enable the
"Fanboy's Social" filter list, Healthchecks does not show
Telegram or WhatsApp icons. This is because the filter list
contains "##.icon-telegram" and "##.icon-whatsapp" entries.

This commit changes the CSS class prefix to "ic-". So we're
now using icon classes like "ic-telegram" and "ic-whatsapp".

As a bonus, we save 2 bytes in HTML per displayed icon :-)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-1066259

Previously, I had changed the default value to "", to force
users to set the SECRET_KEY value (the app refuses to start
if SECRET_KEY is empty).

The problem with that is, out of the box, with the default
configuration, the tests also don't run and complain about the
empty SECRET_KEY.

So, a compromise: revert back to the default value "---".
At runtime, if SECRET_KEY has the default value, show a  warning
at the top of every page.

revert gitignore file

fix remove a space

fix french locale

Add .mo for the French translations