Blur images in invitations from unknown users

When a user receives an invitation from a user that they have never seen before, we should be a bit careful.

The user avatar and/or the room avatar might be offensive or gross, and nobody wants to see that.

So, by default, when we receive an invitation from some user who is not in any of our rooms, we should blur both images. We should also give the user a way to gradually reduce the blur effect to see the full image.