Drop the torbutton submodule in 12.5
We want to rework torbutton (#40938 (closed) 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.