Sharing: use the Site Logo Theme Tool and the Site Icon as fallbacks for Image tags

Until now, if you didn't insert any image into a post, or if you were looking at your home page, Jetpack used a default Open Graph Image tag, and no Twitter Card Image at all.

Now that we've added 2 new ways to define a Site Image ([Site Logo](http://jetpack.me/support/site-logo/) and [Site Icon](http://jetpack.me/support/site-icon/)), I think it'd be nice to use these as fallbacks for Social Meta Tags.

This Pull Request adds fallback for both Twitter and Open Graph meta tags, in that order:
- Blavatar (existing)
- Site Logo (new)
- Site Icon (new)
- Blank image (existing)

Fixes #1191