Support download access by PRIVATE-TOKEN header

Currently there is no way to download a raw file without embedding
the token in the URL, which exposes the token in the URL. There
should be an way of sending this information via the header as the
API does.

Closes https://github.com/gitlabhq/gitlabhq/issues/8137

CHANGELOG

@@ -6,6 +6,7 @@ v 8.5.0 (unreleased)
   - Add "visibility" flag to GET /projects api endpoint
   - Ignore binary files in code search to prevent Error 500 (Stan Hu)
   - Render sanitized SVG images (Stan Hu)
+  - Support download access by PRIVATE-TOKEN header (Stan Hu)
   - Upgrade gitlab_git to 7.2.23 to fix commit message mentions in first branch push
   - New UI for pagination
   - Don't prevent sign out when 2FA enforcement is enabled and user hasn't yet

app/controllers/application_controller.rb

@@ -60,6 +60,8 @@ class ApplicationController < ActionController::Base
                    params[:authenticity_token].presence
                  elsif params[:private_token].presence
                    params[:private_token].presence
+                 elsif request.headers['PRIVATE-TOKEN'].present?
+                   request.headers['PRIVATE-TOKEN']
                  end
     user = user_token && User.find_by_authentication_token(user_token.to_s)