[Feature] Implement persistent cloud storage

[Jack-Edwards]

Description

I would like to implement a means of storing messages and files indefinitely. This feature would be limited to users who have created an account.

This issue relates to #573

Additional Info

This issue should focus on paid cloud storage. Free storage should be a secondary concern, at least for the purposes of this issue.

This issue requires provisioning the storage on the backend, writing the code to handle the new workflows, and implementing a payment system.