fix: Handle newsletter unsubscribe

There were a few issues with the newsletter unsubscribe links that we
sent out in the newsletter. They were pointing to the backend for one
thing.

Also updated the frontend pages to handle unsubscribe from both users
and subscribers.

scripts/newsletter-lib.mjs

@@ -115,7 +115,9 @@ const send = async (test = true) => {
       if (l > 0) {
         const body = mustache.render(template, {
           ...i18n[lang],
-          unsubscribe: `${backend}newsletter/unsubscribe/${sub.ehash}`,
+          unsubscribe: `https://freesewing.org${
+            lang === 'en' ? '/' : '/' + lang + '/'
+          }newsletter/unsubscribe?x=${sub.ehash}`,
           content,
         })
         console.log(`[${lang}] ${l}/${subs} (${i}) Sending to ${sub.email}`)