[IMP] base/web: store more precision digits for rate field

Currently, only 6 digits after decimal point is stored in database and for some currencies
it may require to have more digits after decimal precision to correctly convert rates,
like MXN -> USD currency. So, we allow maximum digits to be stored in database.

On the view side, wherever rate is editable(form or tree view), we show 12 digits
after decimal point and on list view, we truncate limit it to 6 digits.

Also, a bug is fixed for web where 'digits' attribute for float field on list view isn't considered.

Also, we have to remove a test case statement which only checks for 6 digits for a float number.

task: 2024668
closes: #34279

Signed-off-by: Josse Colpaert <jco@openerp.com>