Transfer existing Jetpack sites to WPCC

When a Jetpack blog appears on the results to sync blogs, we want to see if it's already added in the app as a self hosted site. If that's the case, this will take the blog and transfer it to the account.

It would be the equivalent of just syncing and removing the previous self hosted, but this will preserve the synced blog objects and local drafts.

If you want to test this, it's highly recommended that a893627a001b51dec380e5ce81f64f2a0de3367a is merged (currently on 4.6, but not on develop)

Also, only useful for now in debug/internal releases: #2695 

Fixes #2775