Add new per-page option for "allow search engine indexing"

[sam-law]

Just like we have a "show this page in navigation menus?" option, we should have a "allow search engines to index this page?".

If they un-tick the option, then either include a meta tag on-page: <meta name="robots" content="noindex" /> or add an X-Robots-Tag: noindex

https://developers.google.com/webmasters/control-crawl-index/docs/robots_meta_tag

This is very useful for when we have a page on a live site we're working on, and we don't want it indexed in results. For development it's easier to add a site-wide meta tag in the template.

[isaacfreeman]

Moved to Basecamp as a feature request