pullrequests: introduce "action objects" for PR creation Inspired by the command design pattern, this attempts the following: * move policy and business logic from controllers into the model, * move validation, authorization and execution logic closer together in the code, * establish a reusable pattern for modelling higher-level concepts (like "create a new PR iteration"), * make error handling more well-defined, and independent of the controller layer, and * provide clear separation between, one one hand, the validation and authorization of a request, and on the other, the actual execution.