Default agent and class

Currently, the default value of class and agent for new transactions is empty. Meanwhile, if we are on the reports, we only can filter by `All` or by particular class/agent. This will be very difficult if we only want to filter for default transactions (Exclude all transactions that have a value on either class/agent). 

I propose that we:

1. Put a default value class and agent. Then later on the filter, we can filter by the default value. OR
2. Have a new capability on filtering which is `exclude transaction`.

However, I think the 1st approach will be better since it will be easier for a user to do the filtering.