Refactoring getWmsLayerFeatureRequestUrl ready for removal

getWmsLayerFeatureRequestUrl now takes server URL and layer name as an argument.
getWmsLayerFeatureRequestUrl renamed to getFeatureRequestUrl.
Removing getWfsLayerFeatureRequestUrl and using getFeatureRequestUrl as their implementations are now identical.
Update WfsDownloadHandlerSpec.js
Merge pull request #1269 from aodn/refactoring_getting_feature_request_urls

Refactoring getting feature request urls