On tablet/mobile, open the datepicker control in the center of the screen and mask the background

Summary:
Ref T11816. We currently try to expand the picker control from the little calendar icon. This works alright on desktop, but not great on tablet/mobile.

On tablet/mobile, center the control on screen instead. Also, mask the background since it can get a bit busy because we can't really control what's under the control anymore.

Finally, move the little popup thing around if the user resizes the window.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16866