Follow-up to #24278 that is slightly less trivial.
* Some were "trivial" usages that were missed in the previous PR because the same file that had at least one other non-trivial usage.
* These involve extra arguments or inheritance but I have checked that they seem correct.
1. Use `this.` instead of `{{action}}` where applicable
2. Use `{{fn}}` instead of `@actionParam` where applicable
3. Use non-`@` versions of class/type/tabindex/aria-controls/aria-expanded
4. Remove `btn` class (it's added automatically to all DButtons)
5. Remove `type="button"` (it's the default)
6. Use `concat-class` helper
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
