Fixed Jetpack ID sync on REST

Added an extra check for `dotComID` before using REST, and enabled
Jetpack REST for internal builds again
Merge branch 'release/4.6' into issue/2705-fix-jetpack-options-sync
Refactor Jetpack REST checks into separate method
Merge pull request #2708 from wordpress-mobile/issue/2705-fix-jetpack-options-sync

Fixed Jetpack ID sync on REST