Threaded messages

[Jamiras]

Replaces the inbox/outbox with threaded messages, wherein a conversation between two users about a single subject can be discussed (similar to forum posts). Most importantly, reply chains are grouped together and it's easy to see both sides of the conversation.

A new message is created the same as before, and supports full shortcode syntax and preview:

This immediately appears in the user's list (there's no longer a distinction between inbox and outbox):

It also appears in the recipient's list (as unread):

The recipient can open the message and reply to it. (the sender can also open it and send additional information).

The sender is notified of the new message as part of an updated thread:

Both users can view the entire thread and update it at any time:

Either user can delete the thread (which removes it from their list, but not the other participant's list).

If the other participant replies again, it reappears in the first user's list.

If both users delete the thread, it's removed from the database.

[luchaos]

I realize that my review got extremely long and I'm not sure I even addressed all things yet. If you want I can take care of those architectural changes. I wasn't expecting changes like this to be made until I refactored another feature to have some reference.

That said, it's wonderful to finally see some scripts being removed from /public. This is definitely going in the right direction imo.

[luchaos]

This is a good template to go by for the other routes that are yet to be refactored 👏