Handle multiple cases of an attribute in Shibboleth session

Due to the fact that we are using multiple attribute authorities,
there are some cases (specifically email) where there are multiple
values provides for an attribute, separated by a semi-colon.

This update handles those instances and takes the first value in
the list.

gracedb/ligoauth/backends.py

@@ -45,6 +45,7 @@ class ShibbolethRemoteUserBackend(backends.RemoteUserBackend):
     create_unknown_user = True
     attribute_map = getattr(settings, 'SHIB_ATTRIBUTE_MAP',
         DEFAULT_SHIB_ATTRIBUTES)
+    attribute_delimiter = ';'
 
     def authenticate(self, request, remote_user):
 
@@ -93,6 +94,8 @@ class ShibbolethRemoteUserBackend(backends.RemoteUserBackend):
         for user_attr, header in cls.attribute_map.items():
             value = request.META.get(header, None)
             if value:
+                # Check if there are multiple entries; if so, take the first
+                value = value.split(cls.attribute_delimiter)[0]
                 shib_user_attr[user_attr] = value
 
         # Update user with attributes from the shib session only if there are