Make a long-term "server" release, similar to what we do now, and a "desktop" release.

This proposal is similar to or incorporates parts of ?DebianInstallerReleasesAreDebianReleases, FixedReleaseDate, PartialReleasesFullRelease and SubProjectPerStabilityLevel.

It is mostly unlike ReleaseWhenReady (except insofar as the "server" release tries to incorporate some leeway time to allow the release to be as ready as possible), DropStableJustUseTesting, ?ReleasesOnlyCore, and ReleasePerSubsystem.

See ReleaseProposals for alternatives.