Use relative URLs for avatars and rich text attachments

These URLs can be problematic or inconsistent, as we might end up with
rich text content with beta URLs for attachments being used in
production, or viceversa. It'll also be problematic when importing or
exporting data between a self-hosted instance and the SaaS version.

app/helpers/avatars_helper.rb

@@ -42,6 +42,6 @@ module AvatarsHelper
   end
 
   def avatar_image_tag(user, **options)
-    image_tag user_avatar_url(user, script_name: user.account.slug), aria: { hidden: "true" }, size: 48, title: user.name, **options
+    image_tag user_avatar_path(user, script_name: user.account.slug), aria: { hidden: "true" }, size: 48, title: user.name, **options
   end
 end

config/initializers/active_storage.rb

@@ -8,6 +8,17 @@ ActiveSupport.on_load(:active_storage_blob) do
   end
 end
 
+ActiveSupport.on_load(:action_text_content) do
+  # Install our extensions after ActionText::Engine's
+  ActiveSupport.on_load(:active_storage_blob) do
+    # Ensure all <action-text-attachment>s have a "url" attribute that's a relative
+    # path (for portability across host name changes, beta environments, etc).
+    def to_rich_text_attributes(*)
+      super.merge url: Rails.application.routes.url_helpers.polymorphic_url(self, only_path: true)
+    end
+  end
+end
+
 # Don't configure replica connections for ActiveStorage::Record.
 # When ActiveStorage uses `connects_to`, it creates a separate connection pool
 # from ApplicationRecord. This causes after_commit callbacks to fire in