Show scope in search suggestions

When you're in a particular section of a project (for example Issues) and you do a search, the returned results are issues but the search box does not let the user know that's what's going to happen because all it says is "Result name contains XXX".

Since the search box unfolds a suggestion box, it could have a short message indicating the scope of the search.

On the other hand, if you search from the general Issues section (sidebar), the results are always projects, which is inconsistent with the previous behavior.

Central issue #18877 (moved)