Web IDE OAuth: Handle relative url root case

MRs:

• Fix relative_url_root Web IDE OAuth issues (!161328 - merged)
• fix: handle relative url case for OAuth (gitlab-web-ide!357 - merged)
• Fix relative_url_root Web IDE OAuth issues (!162889 - merged)

Description

QA spec for relative urls are failing for Web IDE when the web_ide_oauth flag is enabled by default (see slack thread).

GitLab self-managed instance can be hosted under a specific path. We need to take this into consideration when building URL's in the FE by concatenating location.origin.

Acceptance Criteria

• Handle this case in all instances where we concatenate Web IDE OAuth paths with location.origin.