Do not show website nav in newsletters

I usually just use the "Send a webpage" option of PHPlist when I send out a newsletter for the blog. It makes the actual emails look a bit weird, though:

Screenshot_from_2021-03-09_15-16-34

I'd like to exclude the nav for the pages I send with PHPlist.

I found out I already did some work on this and I'll need to send the current blog post to the newsletter, so I'm adding this to %0.6.0

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