When initializing a new harvest from a planting (via `planting_slug`
or `harvest[planting_id]`), `HarvestsController#new` now properly looks
up the planting, assigns `@harvest.planting` and `@harvest.crop` from
the planting's crop, and sets `@planting` and `@crop` instance variables
for view rendering.
Regression tests added to `HarvestsController` spec and request spec.
Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>