Handle broken image in the link preview card

Addresses #94

Hides the image from the link preview card if the image is not available (the url field is empty) or the status contains sensitive attachments (copying this logic from PleromaFE).

The next step is to handle the scenario when the url is provided but the image can't be downloaded. I'm thinking it might be better to show a fallback image with the same dimensions instead of hiding it since hiding causes the layout jumping and the associated performance issues.