[Enhancement] Implement an ephemeral key exchange between users

[Jack-Edwards]

Key exchange already occurs for every file and message upload.

If a user is not logged in when they upload something, then an ephemeral key pair is created. The public key is uploaded to the database right next to the uploaded item.

If a user is logged in, key exchange is performed with their existing key pair. That user's public key is already stored in the database.

What is missing is the option for users to generate a new key pair for each person they communicate with.