Issue 435 issues category

Ref: https://github.com/OpenUserJs/OpenUserJS.org/issues/435

* Added new category 'Issues' in forum;
    * Sub-forum is accessible from [http://localhost:8080/issues](http://localhost:8080/issues).
    * Description is 'Discussions on scripts'. I'm open to a different/better description.
    * One can not post a new discussion from within the sub-forum (button is disabled).
    * Category column is back in this sub-forum, for linking to script's issues page.
* Removed all discussions with category issue from main forum.