hooks: restore Internet Explorer 11 compatibility.

Compatibility with IE11 regressed in 23eab79946 while working for #3488.
That commit made use of modern js syntax, not supported by IE11.

- Removed arrow functions, replaced with normal functions.
- Removed the spread operator (<...iterable>) and the "new Set()" construct,
  replaced with _.uniq()

At some point IE11 compatibility will be dropped.
Ditching it now, for such a small gain, is not wise.

Fixes #3500.

src/static/js/pluginfw/hooks.js

@@ -143,11 +143,11 @@ exports.clientPluginNames = function() {
     return [];
   }
 
-  var client_plugin_names = [...new Set(
+  var client_plugin_names = _.uniq(
     exports.plugins.parts
-      .filter(part => part.hasOwnProperty('client_hooks'))
-      .map(part => part['plugin'])
-  )];
+      .filter(function(part) { return part.hasOwnProperty('client_hooks'); })
+      .map(function(part) { return part['plugin']; })
+  );
 
   return client_plugin_names;
 }