Add `make format-trailing-ws`. This recipe finds all _files_ (not
links) known to Git and uses `sed` to remove trailing whitespace.
Signed-off-by: Jake Mercer <jake.mercer@civica.co.uk>
Changes after running `make format`. Future commits which add
whitespace should be caught by CI at the PR stage.
Signed-off-by: Jake Mercer <jake.mercer@civica.co.uk>