Support price directives for import

[ppetru]

When importing with an importer that generates price directives, frontent API data validation fails:

https://github.com/beancount/fava/blob/355173882dace6ac9d73b6f8670eb5caf86e48e2/frontend/src/entries.ts#L180

It would be useful to allow importing explicit price directives.

[yagebu]

Sounds good, a PR implementing this would be welcome :)