add /friends endpoint

backend/toolshed/serializers.py

@@ -0,0 +1,13 @@
+from rest_framework import serializers
+from authentication.models import KnownIdentity
+
+
+class FriendSerializer(serializers.ModelSerializer):
+    username = serializers.SerializerMethodField()
+
+    class Meta:
+        model = KnownIdentity
+        fields = ['username', 'public_key']
+
+    def get_username(self, obj):
+        return obj.username + '@' + obj.domain