Fix redirect when accepting invitation using relative URL

What does this MR do and why?

• uses the default rails path helpers(which already contains the relative url) and remove the adding of root_url to it which caused the bug.
• we do not need to provide full URL at this point since the user is in the app, use of _path helper over _url will be ok since the redirect happens in the application.

Screenshots or screen recordings

Exactly as described

How to set up and validate locally

1. Make your gdk operate with a relative path by setting relative_url_root in your gdk.yml to /gitlab (from https://docs.gitlab.com/ee/install/relative_url.html#enable-relative-url-in-gitlab) and restart gdk.
2. Follow steps here

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390603 (closed)