Enable per-project email notification setup

[mattock]

This change allows defining the notification email on a per-project basis.

[mattock]

This was requested by @cron2 some months ago. The current implementation is done with the Scheduler "owner" property:

• https://docs.buildbot.net/3.11.1/manual/configuration/schedulers.html

The Scheduler "owner" property is copied to the Build "owners" property when launching the build. This can be seen from the Build properties pane.

Note that Buidlbot the documentation is wrong - you can't give the owner property a list:

• owner error: builtins.TypeError: unhashable type: 'list' buildbot/buildbot#5380

If the "owner cannot be a list of email addresses" is a problem, we can resolve it in a number of ways:

• Attempt to override the "owners" property of the Build
  • This seemed not to be possible, but is worth another shot
• Add make local MTA (e.g. postfix) forward emails to multiple addresses
• Send emails to mailing lists, where applicable
• Send emails to an email address that can be configured to forward the emails to multiple addresses

[mattock]

I sent a trivial documentation fix to the Buildbot project. Hopefully that gets merged soon so that nobody else needs to spend time figuring out what "owner" property needs to look like.