Location thumbnails rework

[EwoutV]

Now, thumbnails for locations are being hotlinked. We don't have any control over the location of these images, so it's easy for these links to break from time to time.

Allowing location managers to upload their own images resolves this issue. It may also be desirable to add the logo of each authority to make locations more recognizable, by dynamically embedding this logo in e.g. the top left corner of each thumbnail.