[completed #73482508] Remove nofollow attribute on Articles

* added `lessonplanet.com` host to whitelist
* to apply changes to all posts should be run the task `posts:rebake`

NOTE* (might occur in the local environment)
I started discourse on port 4000 on local and "nofollow" was always added because [Discourse.base_url](https://github.com/LessonPlanet/discourse/blob/development/lib/pretty_text.rb#L186) raised an InvalidURIError (`URI::InvalidURIError - the scheme http does not accept registry part: localhost:4000:3000`) which is rescued by adding nofollow to links.