R1:21bd596d7103

Allow draggable elements to be dragged to the first list position

Summary:
Currently, draggable lists (in Config and ApplicationSearch, for example) don't let you drag an item into the first position.

This is because the behavior is correct in Maniphest: the first position is above an initial header, like "High Prioirty", and shouldn't be targetable.

Permit the behavior in general; forbid it in Maniphest.

Test Plan:
- Dragged elements into the first position in ApplicationSearch.
- Failed to drag…