Check related checkboxes

[charsbar]

Should fix part of #363. This PR should allow people to check (at most) three related checkboxes with one check. Maybe we can go further if the issue noted in #427 is fixed.

[charsbar]

It might be better to add the same code to the reindex page where tarballs and meta files are listed together, though I'm not sure if we need to show meta files in the first place. -> cf. #468

[andk]

Thanks, applied and deployed just now