What does this MR do?
This makes sure the Open/Close button on an issuable page uses a relative URL.
This avoids problems when gitlab is used behind a proxy and the URL specified in
gitlab.rb isn't the one used to access gitlab.
Why was this MR needed?
To avoid issues CSRF issues when gitlab is used behind a proxy.
Does this MR meet the acceptance criteria?
Changelog entry added, if necessary
Has been reviewed by Backend
What are the relevant issue numbers?
Closes #35822 (closed)