Fix publishing of Fluent Contents items to include related placeholders & content items

[jmurty]

When a Fluent Contents model that is not a Fluent Contents Page is published the related content items contentitem_set and placeholder_set are not properly cloned to the published copy.

This is because the model-cloning implementation makes some assumptions about how to clone these items, which work for pages but not for non-page models matching the Fluent Contents form, i.e. with layout, contentitem_set, and placeholder_set attributes.

See the related issue AoT 67

[cogat]

@jmurty please can you review the docs, specifically

https://github.com/ic-labs/django-icekit/blob/feature/22-fix-publishing-of-fluent-contents/docs/topics/publishing.md#make-custom-publishable-models

and

https://github.com/ic-labs/django-icekit/blob/feature/22-fix-publishing-of-fluent-contents/docs/howto/page-types.md#abstract-page-type-mixins

for any changes that may result from the rearrangement?

[jmurty]

@cogat I have updated the docs, and made some further tweaks to ICEKit so it provides the base classes necessary to easily subclass fluent content models and admins without the need to re-define identical classes in each project.

Providing these base classes results in some breaking changes for downstream projects – now also documented in the changelog – but I think the pain is worth it make working with fluent contents items much cleaner while we still have the chance.