Allow marking customer/conversation as "priority" from chat widget config

[reichert621]

Problem We don't really use the "priority" inbox much because we often forget to mark important conversations. If we could do this programmatically from the widget, it would be really helpful for us.

Solution Add a priority boolean field to the chat config?

Describe alternatives you've considered TBD

Additional context This is mostly just for ourselves for now :)

Testing Will require some manual testing.

Questions, or need help getting started? Feel free to ask below, or ping us on Slack :)

(You can also check out our CONTRIBUTING.md)