Task Manager; Drilling down to filtered out Tasks is now prevented

When logged in as a User with User Group filters or an External Party User, certain Tasks may be filtered out, e.g.:
* User AB has an Account Log with an Action Date that is allocated to them. This therefore appears as a Task
* The Account with which the Log is associated is filtered out for User AB due to User Group filters

In these cases, the Task preview shows the message 'You do not have permission to view this Task'.

However, the drilldown button in the Tasks grid still allows the Account Log to be open but, when the form is displayed, it fails to load the Log and leaves itself in an unstable state.

The Tasks grid has been updated for filtered out Tasks as follows:
* No drilldown buttons will be displayed
* Double-clicking the row will do nothing for filtered out Tasks