[REF] stock_account: move away from the replay everything strategy

The approach we tried with the current implementation lead to a replay
of all the next moves since a move had to be corrected, and posting an
account entry with the valuation difference.

This proved to be slow and complicated, and moreover the resulting
account entry could be very difficult to understand and justify.

So we moved away from this strategy and implemented the following points:

For FIFO
* If a move goes into negative (sent more than availabale, we do not
  counterpart directly at the next in move: we valued it at the last
  fifo price and only when running `_fifo_vacuum` (which is called at
  every openning of the valuation report), we use some units currently
  in stock and post an account entry with the difference between the
  fake value and the real one used.
* When editing a done move and increasing its quantity, the new quantity
  is available now and will be used at the next move out. We do not try
  to replay the history to make the new values go away directly.
* When editing a done move and decreasing its quantity, it acts as a
  return to the supplier at the price unit of the move.

On average price:
* Don't replay anything, if we fall under the negative stock usecase,
  value at the last average price.
We restore the previous code that set the average price on the standard
price and we slightly patch it to continue to update the average price
if quantity in stock is negative.

We also add a link between stock moves and account moves. A stat button
on the stock move's form redirect directly to them. It's therefore a
little easier to understand what happened.

We remove the separate fifo and average price fields that were added in
saas17, we keep the single standard price (that should be renamed
cost_price for consistency). In the same idea, remove the cumulated
value that was stored on the move. If we need it, just recompute it
(it's not a frequent operation). We remove last_done_move_id,
last_done_remaining_qty and last_done_qty that were added in saas17 to
help in the replay method, as we just got rid of it. We also remove the
code that compensate the negative stock directy at the next in.

What the fifo vacuum does is essentially the same thing as what an out
move do: find a candidate and take units from it. In this case, the
candidates are newer than its concerned stock move. We stored on the
concerned stock move the value at which we sent it (remaining_value) and
post the difference with the real value on a real product.

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