Update RGA validator (#3134)

Schemas/rga_validators.py

@@ -4,6 +4,8 @@ import validators
 class License(Validator):
     tag = "license"
     licenses = [
+        "CC-BY-3.0",
+        "CC-BY-4.0",
         "CC-BY-SA-3.0",
         "CC-BY-SA-4.0",
         "CC-BY-NC-3.0",
@@ -11,7 +13,8 @@ class License(Validator):
         "CC-BY-NC-SA-3.0",
         "CC-BY-NC-SA-4.0",
         "CC0-1.0",
-        "MIT"
+        "MIT",
+        "Custom" # implies that the license is described in the copyright field.
         ]
 
     def _is_valid(self, value):
@@ -21,4 +24,5 @@ class Url(Validator):
     tag = "url"
 
     def _is_valid(self, value):
-        return validators.url(value)
+        # Source field is required to ensure its not neglected, but there may be no applicable URL
+        return (value == "NA") or validators.url(value)