Active issue assignees will now have a button on issues to explicitly
'signoff' their assignment - that is, to affirmatively declare it done.
The use of this new action will allow assignees to signoff without
needing to close the issue as well.  This is useful for multiple
assigned users collaborating on an issue.

Note that if an assignee chooses to close a ticket directly, they are
still automatically signed-off, as before.

To aid this feature, the low-level definition of an 'assigned user' is
updated to exclude users occupying an assignment slot, but which have
already signed-off.

Since the icon representing this action is the same as for closing a
ticket, the buttons enabling these two actions are now color-coded
wherever they appear.

Signed-off-by: Malfurious <m@lfurio.us>

This function helps further abstract agents.  We want to get all users
belonging to a pad that is owned by a group, or more specifically - an
agent.  If this agent is a user, that user is our only user to collect.
If this agent is a group, we want to capture _it's_ owner along with all
of it's members.

Signed-off-by: Malf Furious <m@lfurio.us>

This is probabally more of an oops than a bug, although was causing
unexpected behavior.

When falling back to checking whether the agent has access to the
object's owner, it was wrongly accessing through $this->owner, rather
than $obj->owner (which is the function argument).  This was probabally
left over from how this function _used_ to be implemented (you would
call on the object and pass in the user).

This is basically a constructor for agent.  The actual type returned is
a contrete agent.

The check that this commit adds to each of these functions enables users
with all permissions on themselves.

If the argument doesn't have an owner, then an access error is thrown
when we try to do ->guid.  Since there is no owner, just return false.
Otherwise, do the comparision as usual.

Adding this as an abstract function to class agent.  Since we will only
be sending emails to stored users (and groups) this makes more sense and
allows us to remove this function from the global namespace as well.