Resolve conflict with Panopoly.

Both `oa_core` and `panopoly_pages` have conflicting dependency: `features[field_base][] = body`.

`panopoly_pages` has a dependency on `panopoly_core` which defines:
`features[field_base][] = field_featured_image`.

Therefore, `oa_core` just needs a dependency on `panopoly_pages`.
This also removes `features[field_base][] = field_featured_image` from `oa_core` and fixes the conflict as `panopoly_pages` has a dependency on `panopoly_core`.