client/posts: check reverse search permissions

cf. #424

client/js/models/post.js

@@ -181,6 +181,9 @@ class Post extends events.EventTarget {
     }
 
     static reverseSearch(content) {
+        if (!api.hasPrivilege("posts:reverseSearch")) {
+            return Promise.resolve();
+        }
         let apiPromise = api.post(
             uri.formatApiLink("posts", "reverse-search"),
             {},