Prerelease site does not point to prerelease demo

[pablobm]

The link to "Try the demo" in the prerelease site (https://administrate-demo-prerelease.herokuapp.com/) points to the current demo, not the prerelease one.

Not sure what's a simple way to solve this right now, so leaving this here for discussion and thinking.

[nickcharlton]

This has long bugged me, it's the same when you're running locally. I regularly find myself on the production site and find myself confused.