plugin: Skip enum value checks with a flag+FIXME.

We've long supported skipping documentation checks by flagging the
transaction. We haven't done the same for enumerated metadata because we
need it less often, and bad values tend to do more damage to reports.

However, occasionally when something very off-process happens, we do need it
as a matter of expediency. So support it.

In order to skip validation of these fields, the plugin requires that the
value start with the string "FIXME". This helps ensure that reports have a
consistent way to detect and warn about unfilled values in flagged
transactions.

meta_expense_type: Refine defaults.

* Default "management" for more accounts.

* There's a good handful of accounts where in past audits, the functional
split has been "Conservancy expenses are management, project expenses are
program." Handle those cases too.

expense_type: Revamp expense-allocation metadata.

* Change the name for symmetry with income-type.
* Standardize on "management" value because that's what it's called in the 990.