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
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
