Email step to support multiple recipients

[keevan]

• Notify on abort supports multiple recipients but separated by commas (Allow multiple emails in Notify on abort #881)
• We want to support at least that, but it might be more desirable to make it easier to manage by a human too, so @Peterburnett has proposed we support "an email per line" in a text area input.
• To cover all common bases, we should support ,, ; and \n (new line) separators.