This commit changes the composer to not be read-only on the server
buffer, which tells the user that they can send commands from that view.
On the server buffer, the placeholder is changed to
"Type a command (see /help)", which indicates to the user that this buffer
only accepts commands, and gives them a hint for how to learn what
commands are available.
Implements: https://todo.sr.ht/~emersion/gamja/38