Implement button to clear feature graphic in app edit

Based on !155 (merged).

For now I implemented the deletion of feature graphics like the one of screenshots and APKs: a separate page opens where the users confirm they want to delete the feature graphic.

I did not implemented a special POST to app edit because this solution seems cleaner and aligns well to what's known in Repomaker. In another commit I will catch the delete events, show a JavaScript dialog and delete the objects by posting with JavaScript in the background to the known views.

Fixes #168 (closed).