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

lib/email/styles.rb

@@ -198,6 +198,7 @@ module Email
       style('code', 'background-color: #f1f1ff; padding: 2px 5px;')
       style('pre code', 'display: block; background-color: #f1f1ff; padding: 5px;')
       style('.featured-topic a', "text-decoration: none; font-weight: bold; color: #{SiteSetting.email_link_color}; line-height:1.5em;")
+      style('.secure-image-notice', 'font-style: italic; background-color: #f1f1ff; padding: 5px;')
       style('.summary-email', "-moz-box-sizing:border-box;-ms-text-size-adjust:100%;-webkit-box-sizing:border-box;-webkit-text-size-adjust:100%;box-sizing:border-box;color:#0a0a0a;font-family:Helvetica,Arial,sans-serif;font-size:14px;font-weight:400;line-height:1.3;margin:0;min-width:100%;padding:0;width:100%")
 
       style('.previous-discussion', 'font-size: 17px; color: #444; margin-bottom:10px;')
@@ -237,6 +238,7 @@ module Email
     def to_html
       strip_classes_and_ids
       replace_relative_urls
+      replace_secure_media_urls
       @fragment.to_html
     end
 
@@ -284,6 +286,23 @@ module Email
       end
     end
 
+    def replace_secure_media_urls
+      @fragment.css('[href]').each do |a|
+        if a['href'][/secure-media-uploads/]
+          a.add_next_sibling "<p class='secure-media-notice'>#{I18n.t("emails.secure_media_placeholder")}</p>"
+          a.remove
+        end
+      end
+
+      @fragment.search('img').each do |img|
+        next unless img['src']
+        if img['src'][/secure-media-uploads/]
+          img.add_next_sibling "<p class='secure-media-notice'>#{I18n.t("emails.secure_media_placeholder")}</p>"
+          img.remove
+        end
+      end
+    end
+
     def correct_first_body_margin
       @fragment.css('div.body p').each do |element|
         element['style'] = "margin-top:0; border: 0;"