Fix testing of email notification with multiple recipients.

[donlorenzo]

Currently, sending test notification emails does not work when there are multiple recipients.
The string containing multiple recipients is not split into a list of recipients.
This PR fixes this.
It uses the same logic (splitting on any of , (comma), ; (semicolon), | (pipe), or (space) ) as the main function of sendmail.py.

[MichaIng]

Thanks, just stumbled over this again when working on #3203. Yeah looks exactly like what I'd have done. I'll merge the other one first (as it is security relevant) and the fix the conflict here to test and merge.

[MichaIng]

Ah wait, I fixed that already: #3170
Forgot about it 😄. So while the setting/inputbox is allowed to contain spaces as well (| and ; are not allowed but fail at the frontend, and with #3203 at the backend as well on attempt to "Apply"), the tooltip says it should be comma-separated. Since this is about the "Test" button only, and does not affect existing configs, I preferred to be strict, to be able to harden the rules at some point. I prefer to be explicit about how separation should be done, hence about which characters are allowed, instead of allowing and handling a bunch of different things.

Hence closing this PR. Let me know if #3170 did not fix it for you.