WooCommerce Install Pages Enhancements

When attempting to install a new page:

* only search for pages (ignore posts and other custom post types)
* search for an existing page with the specified page content (typically a shortcode), or if no page content is specified, search for an existing page with the specified page slug.

Should fix #3100 and will also help ensure that WooCommerce only attempts to reuse existing pages that have the WooCommerce shortcodes on them, thus reducing conflicts with other eCommerce plugins.
Merge pull request #3213 from om4james/installpages

Install pages enhancements to help reduce conflicts with other eCommerce plugins.