Fixed create label form not working on issue boards in a sub-group project

Phil Hughesrequested to merge
issue-boards-sidebar-create-new-label-404-error into master

What does this MR do?

Fixes the create new label form in the label dropdown in a sub-group projects issue boards. It was using the path instead of the full_path & was creating a 404 request error.

What are the relevant issue numbers?

Closes #31862 (closed)

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