Delete a container registry asynchronously
When a container registry has many tags, it's easy for the DELETE call to take more than 60 seconds and fail. This can also leave the registry in a bad state with null bytes since some of the images have been deleted with tags still pointing to them. In addition, we have to prevent users from accidentally initiating the delete multiple times or this could leave the registry with orphaned tags.
This commit also adds a flash message to notify the user the registry is scheduled for deletion.