Add a legend to the map

Add a legend to the map that explains the marker size (magnitude) and color (time). Check the current interface for implementation details. We are currently creating a legend that is styled using css. We will need to check the new implementation to make sure that our marker sizes are consistent. I believe that this is no longer done through css.

This will require making a legend.component and adding that to the map similar to the zoom-to.component.