Invoice only direct charges can overencumber a fund without notice

[Tiffany Little]

Semi-related to this bug: https://bugs.launchpad.net/evergreen/+bug/1886583

If you create a direct charge on an invoice that would overencumber a fund, the system will allow you to create and then close that encumbrance without notice.

To test on the Concerto database:
1. On the RF fund, set the Stop Limit to 100 and the Warning Limit to 70.
2. Create an allocation for 10.00 so the fund has a balance of 10.00.
2. Create a blank invoice.
3. Add a direct charge and use the RF fund. Make the amount as 15.00.
4. Save and close the invoice.
5. Investigate the RF fund, and you'll see that it's at -15.00 even though it had a stop limit.

This should be fixed anyway, but when combined with the related bug it means that users don't have an easy way to see if funds shouldn't be used because they're already fully encumbered/spent, and if they *do* create the direct charge and use one of those funds then the system doesn't warn or stop them, it just creates the debit.