FEATURE: treat theme_uploads as settings in JavaScript

This change allows themes and components access to theme assets.

This means that inside theme js you can now get the URL for an asset with:

```
settings.theme_uploads.name
```

app/models/theme.rb

@@ -430,6 +430,16 @@ class Theme < ActiveRecord::Base
       self.settings.each do |setting|
         hash[setting.name] = setting.value
       end
+
+      theme_uploads = {}
+      theme_fields
+        .joins(:upload)
+        .where(type_id: ThemeField.types[:theme_upload_var]).each do |field|
+
+        theme_uploads[field.name] = field.upload.url
+      end
+      hash['theme_uploads'] = theme_uploads if theme_uploads.present?
+
       hash
     end
   end

lib/stylesheet/importer.rb

@@ -63,6 +63,7 @@ module Stylesheet
       end
 
       theme&.included_settings&.each do |name, value|
+        next if name == "theme_uploads"
         contents << to_scss_variable(name, value)
       end
 

spec/models/theme_spec.rb

@@ -546,6 +546,18 @@ HTML
     expect(messages.first.data.map { |d| d[:target] }).to contain_exactly(:admin, :desktop, :desktop_theme, :mobile, :mobile_theme)
   end
 
+  it 'includes theme_uploads in settings' do
+    Theme.destroy_all
+
+    upload = Fabricate(:upload)
+    theme.set_field(type: :theme_upload_var, target: :common, name: "bob", upload_id: upload.id)
+    theme.save!
+
+    json = JSON.parse(cached_settings(theme.id))
+
+    expect(json["theme_uploads"]["bob"]).to eq(upload.url)
+  end
+
   it 'handles settings cache correctly' do
     Theme.destroy_all