Override the button_draft method, in order to
reset the expense and expense_sheet state
when resetting a move to draft.

Steps:

- Create an expense
- Submit and approve the report
- Post journal entry and register payment
- Reset the JE to draft
-> The expense and expense sheet states
   remain the same as before resetting
   to draft. They should be respectively
   'approved' and 'post'

opw-3279364

closes odoo/odoo#120286

Signed-off-by: John Laterre (jol) <jol@odoo.com>