User Message Event Handler

[terrypacker]

Design/Implement the concept of a User Messaging Event Handler.

The idea is that this would be specific to a User so they can customize what event types they want to subscribe to get email/sms messages from.

[terrypacker]

This would use the message transport. For example a User could select to receive all events above Information level via SMS, all information level events via Email/Slack.

This implies a view where a User can see all event types and subscribe to them via some message transport (sms/email/slack...)

[terrypacker]

@jazdw this issue is here to review the Message framework/api implemented for Mango 4.0. We are not necessarily implementing the event handler for 4.0 but would like to be able to after the release so cleaning up this api should be done before the 4.0 release. See com.infiniteautomation.mango.io.messaging package and sub packages for the implementation in the core.