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/mailers/user_notifications.rb

@@ -349,13 +349,26 @@ class UserNotifications < ActionMailer::Base
   end
 
   def email_post_markdown(post, add_posted_by = false)
-    result = +"#{post.raw}\n\n"
+    result = +"#{post.with_secure_media? ? strip_secure_urls(post.raw) : post.raw}\n\n"
     if add_posted_by
       result << "#{I18n.t('user_notifications.posted_by', username: post.username, post_date: post.created_at.strftime("%m/%d/%Y"))}\n\n"
     end
     result
   end
 
+  def strip_secure_urls(raw)
+    urls = Set.new
+    raw.scan(URI.regexp(%w{http https})) { urls << $& }
+
+    urls.each do |url|
+      if (url.start_with?(Discourse.store.s3_upload_host) && FileHelper.is_supported_media?(url))
+        raw = raw.sub(url, "<p class='secure-media-notice'>#{I18n.t("emails.secure_media_placeholder")}</p>")
+      end
+    end
+
+    raw
+  end
+
   def self.get_context_posts(post, topic_user, user)
     if (user.user_option.email_previous_replies == UserOption.previous_replies_type[:never]) ||
        SiteSetting.private_email?