FlexObjects compatibility: change references to Page class to use PageInterface

[paulhibbitts]

Hi @Sommerregen ,

I have started to use the newest Grav 1.7 Beta with Flex Objects, and have discovered a compatibility issue with External Links:
trilbymedia/grav-plugin-flex-objects#16

It looks like the fix for this is fairly minor issue to address (as Andy Miller just did for ShortCodes):
getgrav/grav-plugin-shortcode-core@c374d81

Might you please consider making this change in External Links? I'd be happy to test for you etc. This would then let all early users of Flex Pages use your Plugin🙂

Thanks very much,
Paul

[mahagr]

Also, make sure the minimum Grav version is >=1.6 inside the blueprints.