Rename master branch to default?

[regexident]

Might be a good idea to also add a note to the README to aid the confused?

^{(Would just have created a PR, but as I'm on mobile right now, an issue will have to do for now.)}

[kiliankoe]

This was already on the todo list 😊

I just changed the default branch to stable and updated the base branch of all open PRs as well to point to that (regarding #98, #134 and #141).

You and @stefanhofman will likely have to check locally that you're using a checkout of stable instead of master now (checking out stable and deleting the local copy of master should suffice).

I am a bit concerned about Weblate, which says it's still successfully pulling from master 🤔 I'll leave this issue open until I see that that still works.

edit: I keep forgetting the icon URL in the readme and on nio.chat points directly to the file stored here 😅 Updated that as well ✅

[kiliankoe]

Weblate is weird, reports everything is fine in one view, but errors elsewhere, whatever. It's updated and now and pulls from stable. I think that's it 👍