[plt-scheme] planet server upgraded

Last night I launched a new version of the planet server in
preparation for the impending release of PLT Scheme 4.0. Here are the
major outward-facing changes:

* in the upper left-hand corner there are two links that say [4.x] and
[3xx]. There is now a "4.x" repository that PLT Scheme version 4.x
clients will consult to find packages, and a 3xx repository for older
clients. Unlike the 2xx -> 3xx transition, a single package version
can be in both repositories simultaneously; no more of the situation
we saw in the 2xx->3xx transition where the same package had two
different version numbers in the two different repositories, which
caused needless difficulty. To bootstrap the system, I've populated
the 4.x repository with all the packages and versions from the 3xx
repository. Package maintainers should note a new 'repositories field
for info.ss where a package's repositories can be listed.

* It is now possible for package maintainers to edit the metadata on
any package version, not just the most recent, and it is possible to
submit a new minor version for a not-most-recent major version of a
package. Click the name of your package in the packages view to edit.
It is possible to change which repositories any given package version
appears in through this interface.

* PLaneT does scribble! The server won't build scribblings for
packages already on the server, because the server doesn't execute any
package code (not just for security reasons, but also pretty major
logistical reasons). However, for new package submissions, the PLaneT
client will look for scribblings files, build them on your computer,
and include the output in the package sent to the server. The server
knows where to look for this output, the result being that for new
packages built with very recent versions of the PLT Scheme planet
client, scribble documentation should appear on the server.


Please let me know what you think!  And if something doesn't seem to
be working right, let me know about that too.

-jacob