CRUD for crop terms

[jgaehring]

Taxonomy terms for crops (ie, the abstract category of a crop, not particular instance or planting) can still only be created and modified by importing board data. Provide a UI for adding, editing and deleting crops.

Significant here will also be the color attribute, which is not being displayed at the moment but is included in the type signature and sample data. Some sort of color picker would be nice, or just a pre-selected color palette as options.