Drop the torbutton submodule in 12.5

We want to rework torbutton (#40938 and related).

I propose we directly copy&paste into the tor-browser.git repository for starters, and stop using it as a submodule.

One of the advantages is that it has many features not strictly related to Tor, such as the drag&drop filter and part of the download protection.

So, one of the first developments we do after that could be splitting torbutton in more commits, etc.