Added failing test for issue
Transfer existing Jetpack sites to WPCC

When syncBlogs receives a new blog, also check on the list of Jetpack
connected sites for a match, and migrate that one

Fixes #2775
Merge pull request #2866 from wordpress-mobile/issue/2775-dont-duplicate-jetpack-blogs-4.7

Transfer existing Jetpack sites to WPCC (4.7)