[metabase-lib] Refactor SaveQuestionModal and friends to Question

These previously passed around `Card`s, now they use `Question`.

The main changes are in `QueryBuilder`'s `handleSave`/`handleCreate`,
and `SaveQuestionModal`.

This also pulls a bunch of logic from `queries/utils/description` that
depends on the internals on `Question.datasetQuery().query` (ie. the
MBQL) into (mostly private) methods on `Question`. The tests for
`description` were moved into `Question.unit.spec.js`.