Removed the link_to_issue and replaced references with link_to_workpackage

This is only the first part of the cleanup-task https://www.openproject.org/issues/1753: There is a lengthy discussion in the ticket (https://www.openproject.org/issues/1418) that outlines which parts are affected by the issues -> work_package change. The #1753-Ticket is an initial try to break this down into manageable tasks. 

This commit here __only_ does the link_to_issue -> link_to_work_package, the rest will eventually end up in sub-tickets of #1753.