Add raw build trace download handler

Related with gitlab-org/gitlab-ce!3767. Must be merged before build page changes.

@jacobvosmaer, @ayufan - can you do a review? More details can be found here: gitlab-org/gitlab-ce#15308 and here: gitlab-org/gitlab-ce!3767

Milestone changed to 8.7

This is change is needed, since we support X-Sendfile.

From: https://about.gitlab.com/2016/04/12/a-brief-history-of-gitlab-workhorse/

We ended up with an especially nice solution for file downloads in gitlab-workhorse, inspired by the mechanism Kamil intended to use in NGINX: X-Sendfile headers. Most of the time when you want to use gitlab-workhorse to make something faster or more robust in GitLab you have to write both Ruby code and Go code. But because Ruby on Rails understands X-Sendfile already, GitLab developers can reap the benefits of gitlab-workhorse for file downloads without writing any Go code

So if I understand it properly I don't need to do any handling in Workhorse? I should only send header X-Sendfile: /pat/to/trace.log as an output of rails controller, right?

In that case we can close this MR :)

Status changed to closed

@tmaczukin in Rails you can just do send_file '/path/to/file'. However, what about compression?

@ayufan did we start gzipping build traces or did that never happen?