RFR: 12: Make the notification sender configurable

[Robin Westberg via github.com]

Hi all,

Please review the following change that makes the sender of push notification mails configurable, instead of using one of the commits.

Best regards,
Robin

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Commits:
 - 8860f6ef:	Make sender configurable, add tests

Pull request:
https://github.com/openjdk/skara/pull/3

Webrev:
https://openjdk.github.io/cr/skara/3/webrev.00

Patch:
https://github.com/openjdk/skara/pull/3.diff

Fetch command:
git fetch https://github.com/openjdk/skara.git 8860f6ef:pr/3