Use gon.gitlab_url instead of location.hostname

Created by: felixfbecker

This is the last blocker for https://gitlab.com/gitlab-org/gitlab/merge_requests/16556.

gon.gitlab_url is the canonical URL that that Sourcegraph would be accessing Gitlab under, not location.hostname.

Refactored globals.d.ts a bit to declare all the globals as actual globals instead of only on Window (they are still on Window with this change).