ENYO-1740, ENYO-1884: switch fsDropbox from manual to `dropbox-js` library

- `fsdropbox-js` was the method used by former Hermes implementation to
  access Dropbox, although it was an old & deprecated version of this
  library.
- The functional status of the Dropbox integration remains the same
  (only Ares > Accounts > Dropbox > Check is implemented).

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>