Change scribble/blueboxes to scribble/signature-box

In racket/drracket#291 and racket/drracket#172, it was decided that "signature box" is a much better term than "blue box", and various parts of drracket's documentation were updated to use this phrase. Scribble's scribble/blueboxes library uses the term "blue box" in its documentation and the names of its exported identifiers.

We should add a scribble/signature-box library that's the same as scribble/blueboxes but with changed names. Then we should deprecate the scribble/blueboxes library and point users to the new library.