FORUM-541: [Forum] UnsupportedOperationException when deleting or adding attachments

Problem analysis:
* During the update of a topic/post, the attachment cache data are inserted to topic/post object.
  These data are fixed-size Lists and therefore don't allow "add" or "remove" operation.

Fix description:
* Use addAll to get post information as resizable ArrayList.
FORUM-537: [Answer] Incorrect search result of category name

Fix description:
* Update search query on viewing categories.
* Add one more condition to check if user is admin before adding category in the list.