Save layer thumbnail
Save layer thumbnail

Ensure layer is saved after assigning thumbnail key.

Connects #303

raster-foundry/raster-foundry,kdeloach/raster-foundry,aaronxsu/raster-foundry,kdeloach/raster-foundry,aaronxsu/raster-foundry,kdeloach/raster-foundry,azavea/raster-foundry,azavea/raster-foundry,aaronxsu/raster-foundry,kdeloach/raster-foundry,kdeloach/raster-foundry,aaronxsu/raster-foundry,raster-foundry/raster-foundry,azavea/raster-foundry,azavea/raster-foundry,azavea/raster-foundry,raster-foundry/raster-foundry