This is to try and make it clearer that this method won't override
initialization settings, and is instead simply to add to the default
settings.

This is a problem that could occur when calling converse.initialize twice

Credit for this work goes to @deleolajide

Fixes #2040

- Render chat content as a <converse-chat-content> component
- Create new component for rendering the message body
- Get rid of `showMessage` method