Permission checks for subtasks end up loading the parent issue from the database

Currently when you do a permissions check of a subtask, the parent issue has to be retrieved (from the database) to also check it's permissions. We should look into if there's any optimisations that we can do here (perhaps we don't actually need to retrieve the whole issue from the db, caching the parent issue for subsequent requests etc).

This issue can cause slowdowns in Greenhopper currently when viewing a planning board with a lot of subtasks, since Greenhopper does a lot of permission checks up front.