Use relative paths for opening and closing issuables

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.

As documented by @stwink86 in #35822 (closed)

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
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #35822 (closed)