Create new label/milestone give 404 on permission denied
Created by: robodude666
When a user who doesn't have permission to create a new label or milestone clicks on the "Create new miilestone" or "Create new label" link next to the milestone/labels input fields on the "New Issue" or "Edit Issue" page, they are redirected to a 404 page instead of a "permission denied" page.
The "new" label/milestone pages should show a meaningful permission denied error when accessed without enough permissions, instead of a 404 to remove confusion (and prevent admins from being contacted unnecessarily that the website is broken.)
Preferably, also, if a user doesn't have permissions to create a new label or milestone, the links shouldn't even show up at all.