Feature/linked time entries on wp pane

Before, the link only existed when the costs plugin was loaded. Now, there is always a link (if the user has the permission).

I had to remove an additional allowed_to statement to get costs working with https://github.com/finnlabs/openproject-costs/pull/114. That statement was redundant anyway.
I guess the value of ``$scope.workPackage.links.timeEntries.href`` would be more appropriate here?