fix: do not show "signed up" notifications in mentions tab (#2203)

Partially addresses #2199

src/routes/_api/timelines.js

@@ -66,7 +66,7 @@ export async function getTimeline (instanceName, accessToken, timeline, maxId, s
   }
 
   if (timeline === 'notifications/mentions') {
-    params.exclude_types = ['follow', 'favourite', 'reblog', 'poll']
+    params.exclude_types = ['follow', 'favourite', 'reblog', 'poll', 'admin.sign_up']
   }
 
   url += '?' + paramsString(params)