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 elements into the first position in Maniphest.

Reviewers: garoevans, btrahan

Reviewed By: garoevans

CC: aran

Differential Revision: https://secure.phabricator.com/D7128