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.
Actually, what I want to see are the time entries of that specific work package (see https://github.com/opf/openproject/pull/2230/files#r20560658). So rather ``/work_packages/:id/time_entries`` than all time entries. I think that in this particular case the test should reflect this purpose.