This commit implements many changes to topic and comments embedding. It
deprecates the class_name field from EmbeddableHost and suggests using
the className parameter. discourse_username parameter has been
deprecated and it will fetch it from embedded site from the author or
discourse-username meta.
See the updated code sample from Admin > Customize > Embedding page.
* FEATURE: Add className parameter for Discourse embed
* DEV: Hide class_name from EmbeddableHost
* DEV: Deprecate class_name field of EmbeddableHost
* FEATURE: Use either author or discourse-username meta tag
* DEV: Deprecate discourse_username parameter
* DEV: Improve embed code sample
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