Add clear Avatar Preview entity controller

[dpi]

Add ability to clear out source image cache manually.

[dpi]

This is also useful as all avatar_preview entities must be deleted in order to uninstall the module

[dpi]

This is also useful as all avatar_preview entities must be deleted in order to uninstall the module

No longer applies as of 8.2 since core now bundles the ability to delete all entities for a content entity type.