Always serve favicon from main GitLab domain so that CI badge can be drawn over it (!20263) · Merge requests · GitLab.org / GitLab FOSS · GitLab

Second attempt at fixing https://gitlab.com/gitlab-org/gitlab-ce/issues/47802. The first attempt in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19810 didn't cut it for reasons described in https://gitlab.com/gitlab-org/gitlab-ce/issues/47802#note_84504838.

cc @koffeinfrei