Implement blur-preview when submissions are hidden until you submit yourself

[JobDoesburg]

Basically the title, just as with BeReal.

To implement the blur in a proper way (we are computer scientists, so we should do this properly), I propose to use a very low-res thumbnail (less 50px maybe, so you really cannot see what's on it) and apply a CSS3 blur filter to it (to make it look nicer and not pixellated)