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 this is a case for angular.js#3516

If this becomes very un-DRY we could also use add an extra directive, I suppose: `linkable-value-field` or such like..