Description field for media styles

Problem/Motivation Media styles and responsive image styles can have a name, but often over time it becomes unclear where what style is used. As a result site builders and frontend developers start adding more and more media styles because changing an existing one might break things... Proposed resolution Add a description field that is visible on the admin pages, where information about the use of that style can be given and updated over time. Remaining tasks User interface changes This is a change in the admin interface. API changes Data model changes Release notes snippet