fix: issue #3504 umlaut encoding

updated slugify middleware to handle umlauts to specification

backend/src/middleware/slugify/uniqueSlug.js

@@ -2,6 +2,14 @@ import slugify from 'slug'
 export default async function uniqueSlug(string, isUnique) {
   const slug = slugify(string || 'anonymous', {
     lower: true,
+    charmap: {
+        'Ä' : 'ae',
+        'ä' : 'ae',
+        'Ö' : 'oe',
+        'ö' : 'oe',
+        'Ü' : 'ue',
+        'ü' : 'ue',
+    }
   })
   if (await isUnique(slug)) return slug