[Board] Add endpoint (authed) to make a post on a thread

[Xazin]

This task is two-part, one is implementing the endpoint, and the other is implementing the command to create a Post.

The endpoint should be authenticated (FirebaseGuard).

The command should take these arguments:

• The User (This will be the author)
• Thread ID
• Subject (This can be empty, in that case, it will be "RE: ${thread.subject"})
• Message

Validation before creating the post in chronological order:

1. Validate the user has permission to create posts in that forum
2. Validate that the current last post in that thread was not also by the user, in that case, they should be told to edit their existing one

[Xazin]

@User623-crypto Let me know if you need any help, or if you've stopped working on this :)