move image activity to its own file

bookwyrm/activitypub/image.py

@@ -0,0 +1,9 @@
+''' an image, nothing fancy '''
+from dataclasses import dataclass
+
+@dataclass
+class Image:
+    ''' image block '''
+    url: str
+    name: str = ''
+    type: str = 'Image'