Use relative path link for <link>/<meta> tags in the <head>

Use relative path link for <link>/<meta> tags in the <head>.

For <link>, there is a lot of auto_discovery_link_tag usage.
For <meta>, see app/views/layouts/_head.html.haml

This is especially important for both Geo and customers using a subpath in their external_url.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15738#note_50077405

Discovered while working on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15738

Keywords for easier search: link, meta, head