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.

plugin: Be more selective about when ! skips validation.

It makes sense to let the bookkeeper skip validations in situations
where the metadata requires information that might not be available
when entered. It does not make sense to skip validations that *must*
be available and affect the structure of the books, like project and
entity.

This commit ensures every plugin hook has a test for flagged
transactions, even for hooks that currently have the desired
behavior where no code changes were required for the test to
pass.

Revise chart of accounts used throughout.

The main impetus of this change is to rename accounts that were outside
Beancount's accepted five root accounts, to move them into that
structure. This includes:

Accrued:*Payable: → Liabilities:Payable:*
Accrued:*Receivable: → Assets:Receivable:*
UneanedIncome:* → Liabilities:UnearnedIncome:*

Note the last change did inspire in a change to our validation rules. We no
longer require income-type on unearned income, because it's no longer
considered income at all. Once it's earned and converted to an Income
account, that has an income-type of course.

This did inspire another rename that was not required, but
provided more consistency with the other account names above:

Assets:Prepaid* → Assets:Prepaid:*

Where applicable, I have generally extended tests to make sure one of each
of the five account types is tested. (This mostly meant adding an Equity
account to the tests.) I also added tests for key parts of the hierarchy,
like Assets:Receivable and Liabilities:Payable, where applicable.

As part of this change, Account.is_real_asset() got renamed to
Account.is_cash_equivalent(), to better self-document its purpose.

tests: Turn tested hooks into fixtures.

This is in preparation for passing configuration to hooks.
That'll be a big change already, so I wanted this to be a
boring diff first.