This is a follow-up to: #2293947: Make "Landing Page" into node + Panelizer (rather than a Page Manager page)

Usually you don't want landing pages to appear in search results, because they are things like the home page, the "Contact us" page, a special deal page for Google Adwords, etc.

This is, of course, changable in search_api, but changing it will cause the panopoly_search Feature to be marked as Overridden!

We should provide our own admin page for changing this, and use a trick like we did with #2276089: Allow configuring file types for "File" widget without overriding the Feature