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`

spec/lib/db_helper_spec.rb

@@ -7,8 +7,6 @@ RSpec.describe DbHelper do
     it 'should remap columns properly' do
       post = Fabricate(:post, cooked: "this is a specialcode that I included")
       post_attributes = post.reload.attributes
-      post2 = Fabricate(:post, image_url: "/testing/specialcode")
-      post2_attributes = post2.reload.attributes
 
       badge = Fabricate(:badge, query: "specialcode")
       badge_attributes = badge.reload.attributes
@@ -19,13 +17,6 @@ RSpec.describe DbHelper do
 
       expect(post.cooked).to include("codespecial")
 
-      post2.reload
-
-      expect(post2.image_url).to eq("/testing/codespecial")
-
-      expect(post2_attributes.except("image_url"))
-        .to eq(post2.attributes.except("image_url"))
-
       badge.reload
 
       expect(badge.query).to eq("codespecial")