Avoid using pluralize on moderation pages (#12589)

Pluralize function from Rails framework does not work with other
languages than English, moreover it does not even work properly with
English [1]. Not that the latest applies to this context, it's just
a sign that we best to avoid this function, especially when there are
more reliable ways.

This commit changes how reports pages generated in order to avoid usage
of pluralize function, replacing it with default translation function,
called with given counter. On top of that, we have to make strings
pluralizable, so have to change locale files.

[1]: https://medium.com/@anna7/b3927de2ca8e#6a60

config/locales/ko.yml

@@ -392,9 +392,6 @@ ko:
       created_msg: 리포트 노트가 성공적으로 작성되었습니다!
       destroyed_msg: 리포트 노트가 성공적으로 삭제되었습니다!
     reports:
-      account:
-        note: 노트
-        report: 리포트
       action_taken_by: 신고 처리자
       are_you_sure: 정말로 실행하시겠습니까?
       assign_to_self: 나에게 할당하기