FORUM-798: [Manage Watches form] Email is not deleted in list

Fix description:
* Problem:
	- Do not clear cache (add category/ remove watch)
	- Wrong return when getForumId(forum/service/Utils.java) when input is categoryId or categoryPath
* Fix:
	- Clear watch item cache in CachedDataStorage.java
	- Update method getForumId(Utils.java).
	- Add more test cases to test this updated.