Disallow linking to deleted objects of any kind that do the Status role
Refactor and generalize StrictLinkACL checking into RT::Record

This lets other record classes start to use StrictLinkACL too, simply by
defining a StrictLinkACLRight function.

Slowly, slowly the RT::Ticket linking methods become less special and
more standard.
Remove RT::Ticket->__GetTicketFromURI

RT::Record->_AddLink checks validity for us now, so the only remaining
callsite is unnecessary.  This does not get a deprecation warning and
stay of execution until 4.4, because it is a private method.
Remove duplicate link validation checks from RT::Record subclasses

_AddLink and _DeleteLink now handle these cases.
Refactor link processing on create, AddLink, and DeleteLink into a role

This also fixes the queue right checked in AddLink/DeleteLink to be
AdminQueue instead of ModifyQueue.  ModifyQueue never existed, and the
original code was incorrect when added by 5d4a3be.
Merge branch '4.2/link-validity-refactoring'