Make Rack::Request use our trusted proxies when filtering IP addresses

## What does this MR do?

This allows us to control the trusted proxies while deployed in a private network.

## Are there points in the code the reviewer needs to double check?

If we want to limit what is impacted, we can do this specifically for the rack_attack request object.

## Why was this MR needed?

Normally Rack::Request will trust all private IPs as trusted proxies, which can cause problems if your users are connection on you network via private IP ranges.
    
Normally in a rails app this is handled by action_dispatch request, but rack_attack is specifically using the Rack::Request object instead.

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17550

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)


\cc @stanhu

See merge request !4958