Allow horizontal teasers to have no link

[timdavison]

Teasers have a link either to that from the link_field or if that doesn't exist, to the content node. However for many teasers imported from Falcon there is no link defined - yet linking to the node for something of type link is clearly the wrong thing to do.

So we need to allow for items of type link to NOT have a link (!) and in this case just suppress the read more link.

This is only for horizontal teasers (because that's whats migrated from Falcon) and only for content of type link. It shouldn't affect existing content from standard Drupal sites.