use localStorage for user-doc redirect

Chrome doesn't allow cookies for local files, so try
localStorage (and fall back to cookies where localStorage
is unavailable)
racket/draw/gif: recognize #xFF extension block

Merge to v6.0
unstable-doc: make dependency on DrRacket docs indirect

A direct reference makes the "gui" package depend on the
"drracket" package, and making the reference indirect avoids
that dependency.

Merge to v6.0
remove "unstable-test" as dependency of the "unstable" package

Merge to v6.0
fix "racket-index" package installed in user scope

When the "racket-index" package is in user scope, then the
the documentation-build process should not try to write to
"doc" in the installation.

Merge to v6.0
doc search: fix manual-title table

Merge to v6.0