Immediate avatar and embed variants

Process variants synchronously on attachment to close the window between
image upload and variant availability, guaranteeing that we won't have
lazy variant processing attempts in GET requests.

Tradeoff is that we do variant processing in upload requests, which is
actually desirable. We're working with images that should take
milliseconds to resize given that we'll already have the file on hand.

References https://github.com/rails/rails/pull/51951

test/mailers/notification/bundle_mailer_test.rb

@@ -22,11 +22,12 @@ class Notification::BundleMailerTest < ActionMailer::TestCase
   end
 
   test "renders avatar with external image URL when avatar is attached" do
-    @user.avatar.attach(
+    blob = ActiveStorage::Blob.create_and_upload!(
       io: File.open(Rails.root.join("test", "fixtures", "files", "avatar.png")),
       filename: "avatar.png",
       content_type: "image/png"
     )
+    @user.avatar.attach(blob)
 
     create_notification(@user)