Add new complete strategy

bookwyrm/thumbnail_generation.py

@@ -0,0 +1,14 @@
+class Strategy:
+    """
+    A strategy that generates the image on source saved (Optimistic),
+    but also on demand, for old images (JustInTime).
+    """
+
+    def on_source_saved(self, file):
+        file.generate()
+
+    def on_existence_required(self, file):
+        file.generate()
+
+    def on_content_required(self, file):
+        file.generate()