FEATURE: Add support for secure media (#7888)

This PR introduces a new secure media setting. When enabled, it prevent unathorized access to media uploads (files of type image, video and audio). When the `login_required` setting is enabled, then all media uploads will be protected from unauthorized (anonymous) access. When `login_required`is disabled, only media in private messages will be protected from unauthorized access. 

A few notes: 

- the `prevent_anons_from_downloading_files` setting no longer applies to audio and video uploads
- the `secure_media` setting can only be enabled if S3 uploads are already enabled and configured
- upload records have a new column, `secure`, which is a boolean `true/false` of the upload's secure status
- when creating a public post with an upload that has already been uploaded and is marked as secure, the post creator will raise an error
- when enabling or disabling the setting on a site with existing uploads, the rake task `uploads:ensure_correct_acl` should be used to update all uploads' secure status and their ACL on S3

app/controllers/uploads_controller.rb

@@ -5,7 +5,7 @@ require "mini_mime"
 class UploadsController < ApplicationController
   requires_login except: [:show, :show_short]
 
-  skip_before_action :preload_json, :check_xhr, :redirect_to_login_if_required, only: [:show, :show_short]
+  skip_before_action :preload_json, :check_xhr, :redirect_to_login_if_required, only: [:show, :show_short, :show_secure]
   protect_from_forgery except: :show
 
   def create
@@ -110,6 +110,17 @@ class UploadsController < ApplicationController
     end
   end
 
+  def show_secure
+    # do not serve uploads requested via XHR to prevent XSS
+    return xhr_not_allowed if request.xhr?
+
+    if SiteSetting.secure_media?
+      redirect_to Discourse.store.signed_url_for_path("#{params[:path]}.#{params[:extension]}")
+    else
+      render_404
+    end
+  end
+
   def metadata
     params.require(:url)
     upload = Upload.get_from_url(params[:url])