Jetpack-Connect: Patch Jetpack to check if the user have initiated the install flow from calypso to redirect them there if required

[johnHackworth]

(this issue needs to mirror one in https://github.com/Automattic/jetpack, to be created)

Once we start storing the calypso-initiated Jetpack install in the user metas (#3534) , we need to change Jetpack so we can detect when an install have been initiated from there, so we can redirect the user back to its WordPress.com account once the install is ready.

The user metas will contain a list of urls of each one of the Jetpack install they have started from Calypso, so if Jetpack find that its current site is on that list, it should redirect the user back to calypso once the site is connected