PLANET-6387 Make import/export functionality work for all post types

[planet-4]

We currently have a relatively advanced import/export functionality that unfortunately is only implemented for the "Campaign" post type, only a small fraction of our content.

Luckily it seems that supporting other types is only a matter of removing the check for the post type. There is a quick draft PR where it seems to work already, except for the image import. Needs more testing.

Reporter: pvincent
Sections: Admin Panel

Potentially affected repositories: planet4-master-theme

[comzeradd]

Done at greenpeace/planet4-master-theme#1447