programmatically get legend from wms

This builds the legend url based on the wms url.  It first looks for a legendUrl field in the description and uses that.  If not found it builds a url using getLegendGraphic. 

This has a problem when the server does not have a style set for the wms data (eg aquifer contours) and it is turned off in the list by setting legendUrl to "".

There are issues with this approach when the initial description is not around (share links), but those have always been there so I think we should address that when we refactor.