Add access check when viewing the search block

See #2345601-3: Block caching causes unexpected redirects when submitting empty search form: we currently display search blocks to all users, regardless of whether they have access to the underlying search page. I don't really think that's on purpose, and I doubt there is much use for that. I think most people will probably be confused by this, or some maybe don't even realize it.
However, the workaround of just setting the block visibility is pretty easy and will be readily apparent for almost all users. Furthermore, some sites might actually want to display the block even to users who can't access the search, to then display a "Please log in/register" or "Please give us money for that" message/page, and if we "fix" the current behavior we would make that much harder.
So, I think I'd like to have feedback on this issue before committing anything.