Fix S3 keys/paths for user avatars

Also fixes the avatars controller to work with all back-ends

app/models/user.rb

@@ -172,14 +172,6 @@ class User < ApplicationRecord
     Base64.strict_encode64(data)
   end
 
-  def avatar_filename
-    return nil unless avatar.attached?
-    data = ActiveStorage::Blob.service.download(avatar.key)
-    hash = Digest::SHA256.hexdigest(data)
-    ext = avatar.content_type == "image/png" ? "png" : "jpg"
-    "#{hash}.#{ext}"
-  end
-
   def avatar_variant(size: :medium)
     dimensions = case size
                  when :large  then [400, 400]