FEATURE: force rebake of all posts with images

This was done to pick up 3 changes

1. New pngquant which will result in much smaller images
2. Placeholder images which are missing from old posts
3. Retina images missing from old posts

Also picks up on Image Magick upgrade which slightly alters resize algorithm.

Rebake trickles per: `rebake_old_posts_count` site setting. (100 per 15 minutes)

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