Fix Open Graph tags on singular post types, and minor search improvements

[benlk]

Changes

• Fixes issue where post excerpt and featured media were not being used for open graph tags on post types that are is_singular() but not is_single(). Pull request # for issue #1602.
• Removes duplicate site title in opengraph tags for non-archive, non-is_front_page(), non-is_singular() URLs. Pull request # for issue #1602.
• Removes search form from global nav bar when on the search page, so that there's only one search form. Pull request # .
• Cleans up the search page when no query has been entered. Pull request # for issue #1603.

Screenshots

GCSE before:

GCSE after:

Why

• Fixes OpenGraph description tags aren't output on pages or attachments #1602, originating from a support request in INN internal website discussions.
• Fixes Google Custom Search results page has off appearance on empty-query page #1603, matching Remove search box from main nav when on the search page. INN/inn#94