modify seedwatch all to actually watch all seeds

[myk002]

Fixes #1941 This PR makes two changes:

• The internal map of plant id -> plant index now only contains plants that have seeds. This will allow us to print an error message instead of succeeding but performing no useful action when a plant ID is passed to the API that doesn't have seeds.
• When seedwatch all is called, we iterate over our map and set targets for all plants that have seeds, instead of just the ones in the hardcoded abbreviation list.

No doc updates in this PR since the docs have been rewritten in the docs branch and we don't need to cause conflicts.