Replace individual rewrites for preview mode with a catch-all

[ascorbic]

With moving static files enabled, we currently generate a forced redirect for every file in preview mode, so that the static file isn;t served. For large pre-rendered sites this generates a massive number of rewrites that causes builds to time out. We should be able to replace these with a catch-all, but would need to do some careful handling of other rewrites so that they don;t get override (particularly the /_next/static one).