refactor(*): use term msg to replace item

- `Msg` or `msg` is now the canonical term for the substructure in a
  room. It includes a `chat` subtype and (in the future) other
  administration subtypes like member joining or leaving.

- `Message` or `message` can used in human oriented context like docs
  and comments, but only when it is unambiguous.

- `message` is not chosen in code because it's hard to type (at least
  for me!), and have ambiguous meaning of:
  - "Human readable text" in context of `ApiError`'s field.
  - "A unit of data transfer, datagram" in context of WebSocket Message.

- `item` is not chosen because it is overly generic.

blahd/src/database.rs

@@ -10,7 +10,7 @@ static INIT_SQL: &str = include_str!("../schema.sql");
 
 // Simple and stupid version check for now.
 // `echo -n 'blahd-database-0' | sha256sum | head -c5` || version
-const APPLICATION_ID: i32 = 0xd9e_8403;
+const APPLICATION_ID: i32 = 0xd9e_8404;
 
 #[derive(Debug)]
 pub struct Database {