Forms generally are bound to models, or are completely custom and devoid

of any real default logic beyond rendering and validation. We have some

forms in Djblets that interface with models or other objects, treating

them like key/value stores using set and get methods. These are

all based on SiteSettingsForm, which is built for SiteConfiguration,

and isn't always the best model to use.
This change introduces a new form, KeyValueForm, which works as a base

for all these other forms, SiteSettingsForm included. It handles the

loading of data from an object, the saving of form data back out to

the object, and load/save blacklists for fields (a feature used by

these other forms). It also handles easily marking some fields as

disabled, another feature used by these other forms.
It's built to be usable with any type of key/value storage backend,

by providing several functions that can be overridden to handle the

getting and setting of keys and their values and the creation/saving

of instances.

Unit tests pass in both Djblets and Review Board.
Manually tested configuring extensions and saving the various site

settings forms.