Allow material quantities to reference multiple material types

Here is the original issue that added the material quantity type with a single-value term reference field: #3196834: Material assets, types, quantities, inventory, and filtering

Note to self: look at how this renders in the form and log view. The form element might look better at the bottom of the quantity form. And the log view shows all the material types on one line, which is hard to differentiate.

FWIW SurveyStack wants to start using this before it is officially included in farmOS so we are altering the bundle field definition in their custom module: https://github.com/mstenta/farm_surveystack/commit/7ff937144e0e3ccb4114c...

This is fixed in my 3.x branch, which will be pushed to origin soon.

https://github.com/mstenta/farmOS/tree/3.x