Delete attached photos when a feature is deleted

[gabriel-bolbotina]

Current situation

When a feature is deleted, its attached photos stay on the device and in the project. This leaves unused files behind, and with the composition relation supported (#2315) more features will be deleted automatically, so more orphaned photos could pile up.

Proposed solution

When a feature is deleted, delete its attached photos as well. This also applies to child features deleted through a composition relation (for example, if a parent feature is deleted, then its children should be deleted as well)

Acceptance criteria

• Deleting a feature deletes its attached photo(s)
• Deleting a parent with composition children deletes the photo(s) of all deleted child features (including nested ones)
• If child features are created while adding a parent, and the parent is discarded without saving, the photos of those child features are deleted too
• Deleting a parent with association children does not delete the children's photos (and neither the children, check acceptance criteria of (Relationship strength "composite" has no effect in the app #2315)
• when discarding a parent feature with composition children, the delete confirmation text mentioned that the attached photos will also be deleted

Related to: #2315