Add support for custom fields in extensions.
This introduces two new hooks: ReviewRequestFieldSetsHook and

ReviewRequestFieldsHook.
ReviewRequestFieldSetsHook allows for the creation of fieldsets, or

sectioned groups of fields on the side of a review request.
ReviewRequestFieldsHook allows for the creation of custom fields in any

fieldset. These fields are stored along with a draft and their history is

tracked in the change descriptions. All fields are also accessible/mutable

through the API by making use of the extra_data support.
Currently, all built-in fields are text-based, but a future change will

allow for more types of fields.

Wrote an extension that made use of different fields, and tested them.