Attach: display a larger drop-target when drag event is triggered over the new post container

[urbansheep]

Interface should allow dragging and dropping files of supported types when dragged into the window and to the “New post” container.

When dragging is detected, there should appear a larger drop-target with label “Drop files here to attach”. After file is dropped over the drop-target, file should be passed to the file input and attached to the new post. If dragging was released/interrupted outside drop target boundary, nothing should happen.

[smallq-git]

Note that this functionality was NOT present in Frf.