This works mostly like invoices: i have to “update” the expense form
to compute its total based on the subtotal and the selected taxes,
although in this case i do no need to compute the subtotal because that
is given by the user.
Nevertheless, i added a new function to compute that total because it
was already hairy enough for the dashboard, that also needs to compute
the tota, not just the base, and i wanted to test that function.
There is no need for a custom input type for that function as it only
needs a couple of simple domains. I have created the output type,
though, because otherwise i would need to have records or “reuse” any
other “amount” output type, which would be confusing.\
Part of #68.
