Convert field_image field type from media to image

Problem/Motivation

field_image is used to provide an image associated with a piece of content for debut_article and several other Debut features.

Currently this field is of type media. This was done in order to provide a parallel developer and user experience between this image field and other fields like field_media which might include images plus other media assets.

However, two key issues spark a reevaluation of this early decision:

• While it's possible to do most of what's desired with image display using a media field, it's way way way more complex than using plain old image fields. Bad developer experience if e.g. when you want to display a new image version you need to create a new image preset, create a new file style, link the image preset to the file style for a particular file type, patch File entity to allow you to display a file field--and so on.
• Media module's media browser widget can be applied to image fields, meaning that for the user the experience is just the same.

Proposed resolution

So, probably, it's time to revert field_image to use the image field type. This would involve:

• Change all field_image definitions (in Debut Article and all other Debut features that include field_image).
• Edit all views displays to remove the old field and use the new one.
• Accept that this breaks backward compatibility--you can't upgrade a site that has an existing field instance of a different field type unless you first delete that field.

Remaining tasks

A lot of somewhat painful reworking.

User interface changes

Ideally, none.

API changes

The existing work in Debut Media will be unaffected. We will still display media directly in certain views.