Restore or replace "+" string portion for hidden topics/replies

[SergeyBiryukov]

[6303] removed the + portion from (+ %d hidden) string.

The issue is that now it creates a false impression that the public count includes those hidden items (it does not).

The negative count is hopefully circumvented by [6302], so I think that portion should be restored. Another option is to replace it with "also" or "and" for clarity.

[johnjamesjacoby]

In 6354:

Hidden: Revert part of r6303, and improve some surrounding code:

• Tiny function-call optimizations
• Use integers for _n() and formatted strings for output
• Type casting, phpdoc, and code formatting improvements

Fixes #3077.