The review request fields codebase is pretty crucial to Review Board,

and one of the primary extension interfaces that companies tend to

use and specialize.
In an effort to modernize fields and help reason about them in other

code that needs to interact with them, this change adds typing

throughout the fields codebases.
Fields are now generics, storing the type of data that can be

represented when loading and saving values.
All rendering functions now have return types of SafeString.
The base functions rendering or serializing change description state

work with Any as the type. This is because, while we have a

somewhat-standard format most fields use to record state in

ChangeDescription.fields_changed, this is entirely at the whim of the

implementation. As such, it's up to the field class to narrow the type

as needed and perform any checks and parsing required.

All unit tests passed.
Tested all the field types on a review request, and a wide variety of

change description renderings.