Improve printing experience

[elbotho]

(follow up of #589)

Problems:

• Lazy loaded stuff does not display if user did not scroll
• Link URLs should show as footnotes

Ideas: Try onbeforeprint

In the issue I mentioned some "research" I did. Even onbeforeprint doesn't look to promising, we could probably get some browsers to work, but not all. Same with the footnotes, we need to check if we could run some JS before the print dialog is open and the print content is set. Should we then just not show the URLs then for now? (Like we do currently)

Extend lazyload implementation

If we change the lazy load implementation to set a state one all of them loaded we could make the UX a bit nicer.

Add a printversion using a flag

[Entkenntnis]

Compared to the legacy system, the frontend isn't too bad.

Some missing features I have noticed:

• links are written out (an interesting idea)
• Serlo-Logo added

Anyway, maybe we can find a way to hide the printing warning after the user scrolled to the bottom.

Just some ideas, priority is low.

[elbotho]

(hide comments)

[Entkenntnis]

Hiding comments is doable, see #869, but the rest is - as far as I can see - not achievable without a separate print version of the page - which would be a bigger feature and I'm not sure if this is currently needed.

For more context, see https://github.com/serlo/serlo.org-rfcs/issues/18