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`

app/models/theme_modifier_set.rb

@@ -12,7 +12,7 @@ class ThemeModifierSet < ActiveRecord::Base
 
   def type_validator
     ThemeModifierSet.modifiers.each do |k, config|
-      value = public_send(k)
+      value = read_attribute(k)
       next if value.nil?
 
       case config[:type]
@@ -39,7 +39,7 @@ class ThemeModifierSet < ActiveRecord::Base
   def self.resolve_modifier_for_themes(theme_ids, modifier_name)
     return nil if !(config = self.modifiers[modifier_name])
 
-    all_values = self.where(theme_id: theme_ids).where.not(modifier_name => nil).pluck(modifier_name)
+    all_values = self.where(theme_id: theme_ids).where.not(modifier_name => nil).map { |s| s.public_send(modifier_name) }
     case config[:type]
     when :boolean
       all_values.any?
@@ -50,6 +50,26 @@ class ThemeModifierSet < ActiveRecord::Base
     end
   end
 
+  def topic_thumbnail_sizes
+    array = read_attribute(:topic_thumbnail_sizes)
+
+    return if array.nil?
+
+    array.map do |dimension|
+      parts = dimension.split("x")
+      next if parts.length != 2
+      [parts[0].to_i, parts[1].to_i]
+    end.filter(&:present?)
+  end
+
+  def topic_thumbnail_sizes=(val)
+    return write_attribute(:topic_thumbnail_sizes, val) if val.nil?
+    return write_attribute(:topic_thumbnail_sizes, val) if !val.is_a?(Array)
+    return write_attribute(:topic_thumbnail_sizes, val) if !val.all? { |v| v.is_a?(Array) && v.length == 2 }
+
+    super(val.map { |dim| "#{dim[0]}x#{dim[1]}" })
+  end
+
   private
 
   # Build the list of modifiers from the DB schema.
@@ -78,11 +98,12 @@ end
 #
 # Table name: theme_modifier_sets
 #
-#  id                       :bigint           not null, primary key
-#  theme_id                 :bigint           not null
-#  serialize_topic_excerpts :boolean
-#  csp_extensions           :string           is an Array
-#  svg_icons                :string           is an Array
+#  id                               :bigint           not null, primary key
+#  theme_id                         :bigint           not null
+#  serialize_topic_excerpts         :boolean
+#  csp_extensions                   :string           is an Array
+#  svg_icons                        :string           is an Array
+#  topic_thumbnail_sizes            :string           is an Array
 #
 # Indexes
 #