Global search should allow users to search for other users

This issue is about adding a search area to retrieve Moodle sites users.

Security issues (privacy basically) need to be considered, so users results are only available to users with access to other users details. I haven't properly checked it, but there are a couple of capabilities involved if I remember correctly, in general I think we are pretty flexible, for example I can search for any user in https://moodle.org/message/index.php and I can point to any user profile by URL https://moodle.org/user/profile.php?id=122326, I just need to be logged in. If we confirm this I don't expect many results returned from the search engine and filtered out by check_access (only when guests are searching)