Reset the uploads on login

Clear out images and sqlite file. Deleting the database file is
much faster than removing items one by one.

Warning: when changing accounts you lose queued images. We might
want to change that in future, if we detect that they're there.
Fix positioning of sequence number

Was hidden on iPad because the cells weren't as wide and it was positioning wrong. :)
Fix regression when no camera available

The take photo button was being explicitly re-enabled after the animation,
but pressing it causes a crash on the simulator or device without camera.
Checking to only enable it when camera's on.