Treat allow blocks in metadata as if they were in config

I just realized https://github.com/simonw/datasette/commit/35deaabcb105903790d18710a26e77545f6852ce moved allow blocks from metadata to config.

Could this result in previously secured instances opening up when the upgrade is applied? If so, it would be better to fail hard (like we used to with plugin configuration prior to #2248).