FEATURE: Allow choice of category when making a PM public (#7907)

* FEATURE: Allow choice of category when making a PM public

Previously it would default to uncategorized, which was not ideal on
some forums. This gives the staff member more choice about what they'd
like to do.

* Make the optional category more explicit

* Joffrey's feedback

test/javascripts/helpers/create-pretender.js.es6

@@ -150,6 +150,10 @@ export default function() {
       });
     });
 
+    this.put("/t/34/convert-topic/public", () => {
+      return response({});
+    });
+
     this.put("/t/280/make-banner", () => {
       return response({});
     });