Add more examples for `struct` in Reference

Please merge to v6.0
adjust check syntax so it can show info before expansion completes
and sometimes when there are syntax errors raised

In more detail, rearrange online check syntax's internal plumbing
so that syntax objects can be handed to check syntax at any point
(via a special log message, see the changes to the docs)
not just when expansion is finished

Use this in TR to get check syntax information even when there is
a type error (and also get the check syntax information processing
started before type checking starts (altho this isn't running
in a separate place, so there is no new parallelism here))
Remove/replace some dependencies on scheme-lib
Minimize xrepl-lib dependencies

XREPL already doesn't declare dependencies on its optional
features such as launching DrRacket or opening the docs. The
consensus seems to be that it's best not to declare these and
to gracefully degrade if they're missing. For XREPL, the error
message could be better, but it currently lets the user keep
going when the `dynamic-require` fails.
scribble/lncs: doc repair

Merge to v6.0
reference: add information on the cost of hashing

Merge to v6.0
Removing singletons from Web Server internals
moving delete button
edit on lose focus
Remove arbitrary code execution exploit from Racket and DrRacket

This is particularly bad with DrRacket's online syntax checking, which
causes opening a file to download and executed aribtrary code.
deleting static s3 content properly
increase pkg test timeout
Removing add tag button when not logged in re mflatt
Explicitly enabling download? and install? on user initiated installs

Forgot to commit earlier :(
Revert "Remove arbitrary code execution exploit from Racket and DrRacket"

This reverts commit cf1755fc173cef39c3c4592011623269735084c0.
Explain how to control whether Planet auto-installation is enabled
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