add more detailed metrics for outgoing mails

What does this MR do?

Adds the amount of recipients to outgoing mail metrics and adds metrics for emails send via libmail. This allows to better estimate the total number of mails and why there were sent, e.g. using Grafana.

For mails send via libmail the template is currently set to libmail (since it's currently not possible to identify the type of the mail inside the function).

How confident are you it won't break things if deployed?

Very

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings