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.
If you use the link from the API response (see https://github.com/opf/openproject/pull/2230/files#r20560658) you could write ``viewable: !!timeHref`` instead of introducing a new variable.