Dismantling Grack::Auth part 1: Git HTTP clients

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/14501

This does not completely get rid of Grack::Auth yet because Git LFS
support is 'behind' it and I would like to not make this MR bigger
than needed.

- changed tests to make HTTP requests instead of calling Rack apps
- added missing test cases for Git HTTP authentication
- moved Git HTTP requests into a 'normal' Rails controller

See merge request !3361