Refactors how conversations are stored in ConversationsProvider.tsx

[rhonsby]

Description

This is part 2 of refactoring ConversationsProvider.tsx. This commit specifically refactors how conversations are stored at the top level into an inboxes bucket. This will make it easier for us to continue to adding new inboxes.

Issue

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

Screenshots

https://user-images.githubusercontent.com/1361509/116147217-722c7000-a6ad-11eb-8d54-affa209f3255.mp4

Checklist

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