Define Default URL for ActiveStorage to fix Mixed Content Error

Using ActiveStorage with external services like AWS S3 for saving and surfing attachments like images is great. The setup is well explained in Active Storage Overview in Ruby on Rails Guides.But if you start using it, you will maybe encounter some problems, which are not that easy to find or debug within production. One of the problems we encountered was a Mixed content related to ActiveStorage. This was not a big problem at first. But after we added an iOS App, which was using WkWebViews, some random redirects to AWS S3 images stopped working. We couldn't even see connections for these images to ActiveStorage in logs, which made it harder todebug.