Add controller for rendering avatars on simple URL

app/controllers/avatars_controller.rb

@@ -0,0 +1,20 @@
+class AvatarsController < ApplicationController
+  def show
+    if user = User.find_by(cn: params[:username])
+      http_status :not_found and return unless user.avatar.attached?
+
+      sha256_hash = params[:hash]
+      format = params[:format].to_sym || :png
+      size = params[:size]&.to_sym || :large
+
+      unless user.avatar_filename == "#{sha256_hash}.#{format}"
+        http_status :not_found and return
+      end
+
+      send_file user.avatar.service.path_for(user.avatar.key),
+        disposition: "inline", type: "image/#{format}"
+    else
+      http_status :not_found and return
+    end
+  end
+end