Make user search autocomplete form configurable (enable/disable)

The UserSearchController features two ways how to search for users:
1) using a auto-complete dropdown menu
2) using a standard form with multiple search fields

While the auto-complete feature is cool, it can be problematic in terms of data privacy. This is specially the case when all users have the same kind of email address or some user identifier property. In such cases the search acts a bit like a wildcard search and reveals data without actually entering it.

The solution is to make this auto-completion field configurable (enable/disable) via the olat.properties file.

TODO:

• Add new properties to olat.properties (enabled for everybody by default):
  usersearch.autocomplete.users=enabled
  usersearch.autocomplete.authors=enabled
  usersearch.autocomplete.usermanagers=enabled
  usersearch.autocomplete.groupmanagers=enabled
  usersearch.autocomplete.administrators=enabled
• Inject configuration to UserSearchController
• Add code to only show autocompleterC when configuration matches