Load User Uploads from the CDN

Currently all user uploads in issues, etc. are not using the already working CDN route. This would improve the loading time greatly (by example: https://assets.gitlab-static.net/gitlab-org/gitlab-ce/uploads/3abaae983993505a6bf5068e6f677cea/Screen_Shot_2017-11-14_at_1.35.05_PM.png)

Based on a Slack discussion the best place would be the UploadLinkFilterin the post processing phase for markdown to base URL's on the asset_host

We have to take into consideration again public/private issues and comments cause there the CDN wouldn't work on those URL's. Also that the image URL's are not in the src attribute but the data-src due to lazy image loading.