[Accounting Category] Create categories to understand intention of Disbursement

We need to categorize Disbursements by their intended purpose. For example:

- Repayment of an expense (e.g. Claude paid by HQ; and HCB disburses)
- Funding of an event/project
- Returning extra funds from an event/project

NOTE: categories should be more generic than the reason I mentioned above, but they still need to capture the intention behind the money movement from an accounting perspective; and be granular enough for us to help organizations understand their finances.

For each category, we need to understand which whether it should be omitted from the Expense & Revenue stats. One idea i have is that each category falls under one of three types:
- Expense
- Revenue
- Internal account transfer

We'd just omit any categories with the internal account transfer type.

Start by locating how Disbursements are represented and how Expense & Revenue statistics classify them. Define generic accounting categories and decide which are Expense, Revenue, or internal account transfer. Done means the category set and omission rules are agreed and reflected consistently in disbursement accounting and statistics.

Written by the indexing model from the issue text.