Brave-browser: Add a setting for blocking Disqus comments

[fmarier]

Disqus is a tracking company. It would be nice to be able to block them easily if one doesn't care about reading comments on websites.

While they are listed on the Disconnect list, they are in the "Content" category which we don't use since it has a big impact on webcompat.

[tomlowenthal]

This seems like it should be another control along with the Facebook/YouTube/Twitter/LinkedIn list?

[fmarier]

Yes, another control in chrome://settings/socialBlocking along with YouTube too (https://github.com/brave/brave-browser/issues/4986).

This will likely be a similar PR to https://github.com/brave/brave-core/pull/3784.

[fmarier]

I added Disqus widgets to https://fmarier.github.io/brave-testing/social-widgets.html.