Ensure all nullable / non-nullable field definitions are as intended

Those yucky exclamation marks haven't been done consistently, and this has thus far been overlooked since loosely typed languages have historically been the only client apps. Bonfire's Elixr UIs are changing that, and such inconsistencies need to be fixed.

• All delete* mutations should return non-nullable values.
• All mutation input arguments should be non-nullable for mutations which accept a single structured payload as an argument.
• All mutations should return non-nullable objects
• Fix AgentRelationship.inScopeOf not being manageable via its create / update mutations.
• For all update* mutations which accept a single structured data payload, all parameters except the id should be nullable. RecipeProcessUpdateParams.processConformsTo is the only known issue with regard to this, but a QA pass would be advisable.

Since it came up in QA, also note that Measure.hasUnit is intended to be nullable, and that Measure does not have an ID field.