Adds inboxes for email, live chat, and slack

[rhonsby]

Description

This commit adds inboxes for email, live chat, and slack. You can navigate to them using the left-side navigation under Channels. It basically works by filtering the open conversations and grouping them by their source.

Issue

https://github.com/papercups-io/papercups/issues/760

Screenshots

https://user-images.githubusercontent.com/1361509/116291660-d826ff00-a762-11eb-8e87-7f5b970543d7.mp4

Checklist

• [ ] Everything passes when running mix test
• [ ] Ran mix format
• [x] No frontend compilation warnings