Fix newlines-to-spaces functionality (#6158)

yay for regexes, amirite

app/javascript/mastodon/actions/notifications.js

@@ -31,7 +31,7 @@ const fetchRelatedRelationships = (dispatch, notifications) => {
 
 const unescapeHTML = (html) => {
   const wrapper = document.createElement('div');
-  html = html.replace(/<br \/>|<br>|\n/, ' ');
+  html = html.replace(/<br \/>|<br>|\n/g, ' ');
   wrapper.innerHTML = html;
   return wrapper.textContent;
 };