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.
@ulferts – **not** a pre-requisite for merging, but I think we should make prefer Object literals to switch statements where possible. A good article on the subject: http://toddmotto.com/deprecating-the-switch-statement-for-object-literals/