Revert "Revert "FEATURE: force rebake of all posts with images""

This reverts commit cb317430a1.
No longer needed per: 70269c7c97

db/migrate/20190103060819_force_rebake_on_posts_with_images.rb

@@ -0,0 +1,16 @@
+class ForceRebakeOnPostsWithImages < ActiveRecord::Migration[5.2]
+  def up
+
+    # commit message has more info:
+    # Picking up changes with pngquant, placeholder image, new image magick, retina images
+
+    execute <<~SQL
+      UPDATE posts SET baked_version = 0
+      WHERE id IN (SELECT post_id FROM post_uploads)
+    SQL
+  end
+
+  def down
+    # no op, does not really matter
+  end
+end