Deleting ad_entity that display relies on deletes the entire display

Problem/Motivation

When deleting one or more ad entity dependencies of a display, the entire display is deleted instead of just setting the breakpoint settings to no ad entity.

Steps to reproduce

Suppose we have three ad entities:

- entity_mobile
- entity_tablet
- entity_desktop

And a display 'entity' that has these entities set for mobile, tablet and desktop breakpoints respectively.

We now decided we do not need mobile and tablet entities any more. We delete them.

The 'entity' display is now also deleted.

Proposed resolution

Don't delete the entire display if an ad entity is deleted that the display relies on. Maybe just set the breakpoint settings for the missing ad entity to "none" or only delete the display configuration if there are really no more entities that can be used in that display configuration.

Remaining tasks

User interface changes

API changes

Data model changes