Move default for 'panels_page_allowed_layouts' variable from panopoly_core to panopoly_theme?

This is something that's been bothering me since working on #2191069: Have Panopoly Theme depend on Radix Layouts (rather than providing it's own)

panopoly_core sets up the default for the 'panels_page_allowed_layouts' variable, which enables all the Radix layouts (previously, it enabled all the Panopoly layouts). However, it's panopoly_theme that brings in the Radix layouts! For distributions that want to use panopoly_core but not panopoly_theme, this is just weird.

This makes more sense in panopoly_theme, but I'm haven't had a chance to think about the update implications associated with it, so that's why I didn't do it as part of that issue... But let's not lose sight of this potential clean up!