Add Advertisment Content type: "Responsive Image"

Problem/Motivation

Banners may need to be responsive. Currently we only have the "Image" Ad content type, but I think a similar type with a responsive image field might make sense.

We might add that in the main module or as submodule.

This should be implemented as soon as someone needs it (and sponsors the development) or prepares a MR.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes