feat: Add Activity export

See #27
See #29
Resolves #24

export/serializers/activities.py

@@ -0,0 +1,52 @@
+from rest_framework import serializers
+
+from activities import models
+
+from . import base
+
+
+class ActivityTypeSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = models.ActivityType
+        fields = [
+            'pk',
+            'name',
+        ]
+
+
+class ActivityMediumSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = models.ActivityMedium
+        fields = [
+            'pk',
+            'name',
+        ]
+
+
+class ActivitySeriesSerializer(serializers.ModelSerializer):
+    type = ActivityTypeSerializer()
+    medium = ActivityMediumSerializer()
+    
+    class Meta:
+        model = models.ActivitySeries
+        fields = [
+            'pk',
+            'name',
+            'type',
+            'medium',
+        ]
+
+
+class ActivitySerializer(base.FlattenedModelSerializer):
+    series = ActivitySeriesSerializer()
+    type = ActivityTypeSerializer()
+    medium = ActivityMediumSerializer()
+
+    class Meta:
+        model = models.Activity
+        fields = [
+           'name',
+           'series',
+           'type',
+           'medium',
+        ]