UpdateFormMixin was introduced in Djblets 0.9 to make it easier for

API resources to interface with forms. It offered functionality for

creating instances of forms and to parse data coming in from the API

request.
It was a very useful first step, but left much up to the API resources.

It required the caller to handle much of the request/response data

handling, had no official method for parsing methods to validate and

raise errors, and didn't handle a form's initial data.
This change offers a whole new set of improvements to this mixin. It

makes it easier to construct forms from API requests by automatically

providing all initial field data for the form, so that the caller

doesn't have to, and it provides official support for gathering and

returning errors from parsing methods.
It also wraps much of the existing functionality in some new convenience

methods. Rather than calling create_form(), API resources can now call

handle_form_request(). This takes care of calling create_form(),

checking for form validity, and then returning an error payload or

saving and returning a success payload.
Subclasses can override how these payloads are constructed, perform

additional error payload handling, change how forms are saved, and

manipulate data before or after creating the form. This also

automatically works with the new field validation in the parser methods.
These changes should slim down any resources that use forms

significantly, and make it much easier to maintain resources in a

standard way going forward.

Unit tests pass.
Used all the new functionality in a resource in Review Board and

saw the expected behavior.