Adds a 'readonly' prop to ChatContainer that conditionally applies a
'readonly-chat' CSS class to the chat container element. This allows
streamers using OBS to target the readonly chat embed with custom CSS.
The readonly embed at /embed/chat/readonly now passes readonly={true}
to ChatContainer, which adds the class to the container div.
Fixes#4266
Co-authored-by: Gabe Kangas <gabek@real-ity.com>
