Merge branch 'haynes/gitlab-ce-remove_access_control_for_images' into 'master'

Remove access control for uploaded images to fix broken images in emails

Replaces !530.

> This MR removes the access control for uploaded images. This is needed to display the images in emails again.
>
> The previous solution to base64 encode the images had to be reverted, because not all email clients supported it.
>
> If possible this should go into the 7.10 release.

See merge request !533