Please add block ABOVE whole checkout

[seansan]

We have programmed setting to

1. remove header in checkout
2. remove footer in checkout

request now is to

1. add a call to a custom block (just like https://github.com/elsner-jugal/snh-settings/issues/6)
2. above the whole checkout (so as an alternative header)
3. that we can turn on/off in backend
4. and add comment to setting + group these options together

See example where headers + footers are gone, but alternative simple header is shown