Show broken flag

[maralorn]

I think we should consider the broken flag in the search index.
One motivation is, that currently the search shows all haskellPackages, but 2/3 of them are broken. That clutters the search results and it is super frustrating for users when they get advertised a package to then realize later, that it does not work.

The imo best suggestion made at #ZurichZHF would be to by default filter out packages marked broken, but show something like "n currently broken packages hidden" then you get to toggle on showing them and they should be somehow marked.

Advanced features would be

• "broken since"
• "build error leading to disabling"
• "link to a Guide how to fix the package"