This splits up reviewboard.admin.forms into multiple files, making it

easier to maintain as new forms and utility classes are added. It

doesn't change any logic or formatting, with the exception of adding

module-level docstrings. An upcoming change will bring these classes

up-to-date with our modern coding and doc standards.
This is in preparation for an upcoming change that will add classes for

helping transition Django's idea of Change Form fieldsets and fields to

what we need for the new admin UI.

Unit tests pass.
Manually tested every settings form that moved.