Update badges on all READMEs

[stefan-k]

Some badges are broken and some are outdated (for instance linking to the old Gitter channel).

[sazuyakun]

@stefan-k
Hi, do we have to replace the Slack links with discord aswell?

[stefan-k]

Thanks @sazuyakun for taking over this issue so quickly. I'll respond to your question via a review of your PR :)