Add healthcheck URL to protect against silent failures

[shukon]

This PR continues #142. If there is anything I can do to help merging, just tell me.

Problem

There is no easy solution for external monitoring of the backups. Since there's nothing worse than setting up a backup to discover after 6 months (or years) that it stopped working after a couple of days.
Yunohost sending mails alleviates the problem somewhat, but it doesn't protect against silent failures.
This PR fixes #117 - it can be extended later on by multiple options for backup monitoring, something like the backup-hooks in borgmatic, but it doesn't have to.

Solution

Introduce an optional config-variable called healthchecks_url that is called upon success or failure (in the event of failure it includes the logs). You can send this either to your own hosted version of healthchecks.io or to the official one.

PR Status

• Code finished and ready to be reviewed/tested
• The fix/enhancement were manually tested (if applicable)
  • -> I tested install with and without URL, with and without failures as well as changing the URL.

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[Limezy]

Wow, that's great, I'll use it with my Ntfy instance