Make Hero Image and Cover Image required for Exhibit and Collection

[foglabs]

Because

Because hero image and cover image are used across the site on various pages, exhibits and collections should require both image fields to be filled.

Done when

• [ ]

[mrharpo]

@foglabs I like the idea, but I'm not sure if it should be a hard requirement for the record. E.g:

if a content editor wants to create a record stub (as a draft) they would have to add both images (or 2 placeholders) before they could save it, even as a draft.

Hooks

Wagtail has a before-publish-page hook that we can use to make sure everything is there before publishing.

With that in mind, can the frontend render a placeholder image (OV logo?) to display on demo for such cases?