Use schemaless URLs for assets
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=20499) </details> <!--IssueSummary end--> There seems to be some valid scenarios where it's desired to have both HTTP and HTTPS connections available for the same GitLab instance, to support legacy systems: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1891#note_36647114 That means we should either use relative URLs or schemaless URLs for the assets. cc @jschatz1
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