Bug 2542: Validate the amount of a manual invoice/create

Bug 2542: Validate the amount of a manual invoice/create

If a manual invoice/credit amount is not correctly set, it should not be
accepted.

Test plan:
Try to create a manual invoice and a manual credit with an amount containing
something else than numbers.
It should not be possible.

Followed test plan. Patch behaves OK, including that negative values are not allowed.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>