Main image in event details view: use the thumbnail version until large image loads

[troughc]

Because the images are large and high-res, they take a bit of time to load when you click to see event details.
This means that (sometimes) for around 1 second, there is no image loaded.
To make the experience better, temporarily load the thumbnail version until the large image is cached.
My understanding is that this feature can be implmented in a way where the user will not notice any difference when the large image loads (other than a quality change). So the different aspect ratios shouldn't be apparent.
cc @saumier