Add icon field to instance endpoint

As we now allow admins to customize their app icon, it may be useful to expose it so apps can use it in the UI.

This is using the Android icons as they provide a good range of sizes.

app/serializers/rest/instance_serializer.rb

@@ -7,10 +7,12 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     has_one :account, serializer: REST::AccountSerializer
   end
 
+  include InstanceHelper
+  include ApplicationHelper
   include RoutingHelper
 
   attributes :domain, :title, :version, :source_url, :description,
-             :usage, :thumbnail, :languages, :configuration,
+             :usage, :thumbnail, :icon, :languages, :configuration,
              :registrations
 
   has_one :contact, serializer: ContactSerializer
@@ -33,6 +35,18 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     end
   end
 
+  def icon
+    SiteUpload::ANDROID_ICON_SIZES.map do |size|
+      src = app_icon_path(size.to_i)
+      src = URI.join(root_url, src).to_s if src.present?
+
+      {
+        src: src || frontend_asset_url("icons/android-chrome-#{size}x#{size}.png"),
+        size: "#{size}x#{size}",
+      }
+    end
+  end
+
   def usage
     {
       users: {