core: frontend: kraken: auto-round app icon corners

[ES-Alexander]

Current behaviour

Some Extensions have pre-rounded corners in the images of the app icons, while some are hard squares, which then looks harsh if the background colour is not the one that's expanded to the edges of the card (raised by Elisa, in SMS).

Expected or desired behaviour

Round the corners of the app icons in the store cards (border radius ~20px): -->

and also in the Extension Details Modal (border radius ~13px): -->

Prerequisites

• [x] I have checked to make sure that a similar request has not already been filed or fixed.