Add discourseEmbedClass option and apply classname to iframe html

[patdavid]

This will allow the embed javascript to include a parameter
discourseEmbedClass that will be passed to the iframe page
where it will be parsed and applied to the iframe element
as a class.

This allows targeting of CSS for different embeds.

[mention-bot]

Thanks @patdavid for your pull request 👍. By analyzing the blame information on this pull request, I identified @eviltrout and @techapj to be potential reviewers.

[discoursebot]

Thanks for contributing this pull request! Could you please sign our CLA so we can review it? http://www.discourse.org/cla

[patdavid]

This is in reference to: https://meta.discourse.org/t/embedding-in-more-than-one-host/62002/

[patdavid]

Could possibly ping @SamSaffron and @ZogStriP as well.

[eviltrout]

I don't like the idea of the client passing this through. It seems like it could easily be used for XSS issues.

It should be part of the embeddable_host model and then inserted automatically by the rails application, plus an admin interface to add the class name to the particular host.

[eviltrout]

(Also I meant to say thanks for the effort! Sorry that came across as rude.)

It just needs to go back to the drawing board.

[patdavid]

Even better! I don't have the depth of understanding required to probably help much in this case the embeddable_host model. :(

Is there something I could do to help with that in mind?

[eviltrout]

I'll post in the meta topic. Maybe someone can offer to help you out.