If set to a positive integer, the chat history will be kept to that size
by removing older messages.
This happens as new messages come in (as long as the chat isn't scrolled up)
and when the user scrolls down.
Also add the `pruning_behavior` setting
More work on making bottom panel sub-components declarative
- Handle auto-completion in the converse-muc-message-form element
- Make message limit indicator a component
- Rename template