Save Site Icon URL into Jetpack Options

This allows the site icon to sync with WordPress.com.
Set up syncing of site icon URL
Change URL to 512px version
Fix typo (incorrect option name)
Fix the jetpack_site_icon_url to return a square when requesting the 512 image
Removal of space
Make sure we have the site_icon_url when migrating from a version that didn't have it.
Rebasing with master
Merge pull request #1371 from Automattic/site-icon-sync

Site icon sync