Rollback to official Dropbox JavaScript API.

It is just too user-unfriendly and error-prone to split an one-step sign
in process into a two-step one. Adopting an OAuth receiver page located
on a free web hosting service would be better.

Change-Id: Iee6a500c931df2a5b8295bf0486aea860470a6c8
Signed-off-by: Wei-Chung Wen <morisato3711@gmail.com>