Move from using a submodule to actual files

This fixes a problem where the toolkit existed in
`./node_modules/govuk_frontend_toolkit/govuk_frontend_toolkit/`
when it was installed, but actually we want it in
`./node_modules/govuk_frontend_toolkit/`.
Add initial copy of toolkit files

These will be updated by Jenkins in future, but this is the initial
commit.
Improve package.json

- Remove unnecessary/wrong fields
- Set author to everybody
Update CONTRIBUTING.md to say how this is updated
Update README.md