Move to GPLv3

We should probably move to GPLv3 for the reasons we talked about, concerning Litecord. Also copyleft is good.

I think it's better to go with GPLv3-only as it protects us against an evil corp taking over GNU Foundation to release a newer GPL with legal loopholes and everything.

So we'll need to:

• add a line to every source file, with "gplv3 or later" wording changed to "gplv3".
• (mark this as not WIP)
• get approval of every code contributor
• (merge)