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.