FEATURE: Include optimized thumbnails for topics (#9215)

This introduces new APIs for obtaining optimized thumbnails for topics. There are a few building blocks required for this:

- Introduces new `image_upload_id` columns on the `posts` and `topics` table. This replaces the old `image_url` column, which means that thumbnails are now restricted to uploads. Hotlinked thumbnails are no longer possible. In normal use (with pull_hotlinked_images enabled), this has no noticeable impact

- A migration attempts to match existing urls to upload records. If a match cannot be found then the posts will be queued for rebake

- Optimized thumbnails are generated during post_process_cooked. If thumbnails are missing when serializing a topic list, then a sidekiq job is queued

- Topic lists and topics now include a `thumbnails` key, which includes all the available images:
   ```
   "thumbnails": [
   {
     "max_width": null,
     "max_height": null,
     "url": "//example.com/original-image.png",
     "width": 1380,
     "height": 1840
   },
   {
     "max_width": 1024,
     "max_height": 1024,
     "url": "//example.com/optimized-image.png",
     "width": 768,
     "height": 1024
   }
   ]
  ```

- Themes can request additional thumbnail sizes by using a modifier in their `about.json` file:
   ```
    "modifiers": {
      "topic_thumbnail_sizes": [
        [200, 200],
        [800, 800]
      ],
      ...
  ```
  Remember that these are generated asynchronously, so your theme should include logic to fallback to other available thumbnails if your requested size has not yet been generated

- Two new raw plugin outlets are introduced, to improve the customisability of the topic list. `topic-list-before-columns` and `topic-list-before-link`

lib/cooked_post_processor.rb

@@ -490,16 +490,26 @@ class CookedPostProcessor
   end
 
   def update_post_image
-    img = extract_images_for_post.first
-    if img.blank?
-      @post.update_column(:image_url, nil) if @post.image_url
-      @post.topic.update_column(:image_url, nil) if @post.topic.image_url && @post.is_first_post?
-      return
+    upload = nil
+    eligible_image_fragments = extract_images_for_post
+
+    # Loop through those fragments until we find one with an upload record
+    @post.each_upload_url(fragments: eligible_image_fragments) do |src, path, sha1|
+      upload = Upload.find_by(sha1: sha1)
+      break if upload
     end
 
-    if img["src"].present?
-      @post.update_column(:image_url, img["src"][0...255]) # post
-      @post.topic.update_column(:image_url, img["src"][0...255]) if @post.is_first_post? # topic
+    if upload.present?
+      @post.update_column(:image_upload_id, upload.id) # post
+      if @post.is_first_post? # topic
+        @post.topic.update_column(:image_upload_id, upload.id)
+        extra_sizes = ThemeModifierHelper.new(theme_ids: Theme.user_selectable.pluck(:id)).topic_thumbnail_sizes
+        @post.topic.generate_thumbnails!(extra_sizes: extra_sizes)
+      end
+    else
+      @post.update_column(:image_upload_id, nil) if @post.image_upload_id
+      @post.topic.update_column(:image_upload_id, nil) if @post.topic.image_upload_id && @post.is_first_post?
+      nil
     end
   end