Deadlines should be supported for multiple add

[tedks]

Currently, to add a set of tasks that all have the same deadline, a user has to use the multiple-add pane to add a group of tasks, and then edit all of them to set a due date. This makes the multiple-add pane not very useful, since you can actually do it faster (and all on the keyboard) by just pasting the same due date for each field and adding them individually, pasting the due date every time.

The multiple add pane should have a due date and show by field that should apply to all the tasks.

[dnrce]

Agreed. I don't see any reason not to have feature parity between the single/multiple panes.

[dnrce]

@tedks is going to work on this one with me.

[lrbalt]

These fields were left out to minimize the amount of fields in the form. I don't see any problem in putting due_date and/or show_from back in.