Credit third-party projects

It's been in the back of my mind for a long time, and I had some energy this morning. This MR introduces a new "/credits" page that lists important projects, initiative, tools, services we use to do our work on Funkwhale.

I think it's important to acknowledge we're part of an ecosystem, and that supporting us can also be achieved by supporting the projects we rely on.

I've added as many projects/links as possible, but obviously we can add more if I forgot anything ;)

You can view the new page live at http://funkwhale.pages.funkwhale.audio/-/funkwhale.audio/-/jobs/31400/artifacts/review/index.html, and by clicking credits in the footer.

cc @Sporiff

Screenshots