Add cover_image to screenshot response in API

app/flatpages/help/api.md

@@ -226,6 +226,7 @@ curl -X DELETE https://content.minetest.net/api/packages/username/name/releases/
         * `url`: absolute URL to screenshot.
         * `created_at`: ISO time.
         * `order`: Number used in ordering.
+        * `is_cover_image`: true for cover image.
 * GET `/api/packages/<username>/<name>/screenshots/<id>/` (Read)
     * Returns screenshot dictionary like above.
 * POST `/api/packages/<username>/<name>/screenshots/new/` (Create)

app/models/packages.py

@@ -1093,6 +1093,7 @@ class PackageScreenshot(db.Model):
 			"url": base_url + self.url,
 			"approved": self.approved,
 			"created_at": self.created_at.isoformat(),
+			"is_cover_image": self.package.cover_image == self,
 		}