[fix bug 1104336] Add option to exclude search engines.

This is consistent with how we handle the other properties, but I wonder if maybe we want to go with a ManyToMany relationship to handle this instead. Then we'd have a SearchEngine model that we can update on the admin interface when we want to exclude other search engines.

Also, if we eventually want to send snippets to users on specific search engines instead of excluding them, a ManyToMany seems easier to adapt then a bunch of booleans on each snippet.

Thoughts?