CKAN Version if known (or site URL)

2.8+ (master)

Please describe the expected behaviour

There is a shortcut for the search legacy route in CKAN 2.8+, I guess it's missing in LEGACY_ROUTE_NAMES.

Please describe the actual behaviour

On master search is not available as route, basically the same as described in #4066

What steps can be taken to reproduce the issue?

h.build_nav_main(
          ('home', _('Homepage')),
          ('search', _('Datasets')),
          ('group_index', _('Groups'))
        )