Show sources without leaving the website

[Betree]

[Betree]

@wdestin implemented a great proof of concept for this feature in https://github.com/CaptainFact/captain-fact-frontend/pull/765. While the prototype worked well for simple cases, dealing with the x-frame-options header and dead links ended up being pretty challenging. The delay introduced pre-fetching the source was actually going to degrade the UX instead of improving it.

If we want to take another look at it, we should have clever specs that take these issues into account. That might however not be a priority at the moment.