Publish old issues of H+M

[lchski]

There are some extended periods where issues exist in the Buttondown archive but aren’t on my site. In most cases, the Markdown is sitting in _newsletters, which we can find with ripgrep:

rg --files-without-match -d 1 -e "---" _newsletters/

For the rest, we may need to extract the published version numbers and diff those from a sequence of 1...n, where n is the latest published issue. To find the numbers (but not actually extract them, would need to further modify the command):

rg -e "^number: " _newsletters/