Move all our review request field logic into new field classes.
This introduces a new set of classes and functions for representing

fields on a review request. These classes are responsible for rendering

the list of fields on the review request page, loading values from the

database, saving from a draft, recording change description info, and

rendering that change description info.
Fields are grouped by Fieldsets. There are three default fieldsets:

"main" (the area for Submitter, Description, and Testing Done), "info"

(Repository, Bugs, etc.), and "reviewers". Additional fieldsets can be

registered, and fields placed in them. These will render on the side,

below Reviewers.
This work allows us to simplify a lot of our template code for the

review request, as well as the change description rendering code. It

also allows us to provide a level of extensibility for fields. Soon,

extensions will be able to easily provide their own fields on a review

request.
There's also a small but measurable impact on load times. By reducing

the work the template has to do, one review request I have with about 10

change descriptions saves about 10ms of render time.

Verified that everything displayed exactly as it did before. This includes

all fields, with and without values, and all combinations of change description

items.
Saved every type of field and published drafts. The new values persisted and

their changedescription entries were recorded and shown.