Fix silence and unsilenced response bodies

Both response bodies had a typo that included suspended_at, so I renamed
it to silenced_at.

app/controllers/admin/users_controller.rb

@@ -350,7 +350,7 @@ class Admin::UsersController < Admin::AdminController
         silenced: true,
         silence_reason: silencer.user_history.try(:details),
         silenced_till: @user.silenced_till,
-        suspended_at: @user.silenced_at,
+        silenced_at: @user.silenced_at,
         silenced_by: BasicUserSerializer.new(current_user, root: false).as_json
       }
     )
@@ -365,7 +365,7 @@ class Admin::UsersController < Admin::AdminController
         silenced: false,
         silence_reason: nil,
         silenced_till: nil,
-        suspended_at: nil
+        silenced_at: nil
       }
     )
   end