Add hard stops on empty batch edit sets
Summary:
Ref T8637. If a user tries to batch edit a list of tasks which can't be edited, we fall through to `withIDs(array())`, which can affect //everything//.
Explicitly stop batch editing if we don't have valid IDs or valid tasks.
The UI sort-of warns you that something is wrong, but this is ultimately a pretty severe UX issue. I'll fix the underlying Query in the next diff.
Test Plan: Tried to batch edit a list of tasks I didn't have permission to edit.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: lloyd.oliver, epriestley
Maniphest Tasks: T8637
Differential Revision: https://secure.phabricator.com/D13388