Convert toolkit to be a real submodule

Currently while there is a `.gitmodules` file the `govuk_frontend_tookit` folder is an actual copy of the toolkit rather than just a submodule reference to it. This removes that copy and uses a real git submodule. This means that none of the submodule files will be actually kept in this repo, instead just a pointer to the current tip of master is kept around.

I also took the liberty of using the `https://github` url rather than the `git@github` so external people could use the submodule.