Split the administration forms into multiple files.

Review Request #10796 — Created Nov. 11, 2019 and submitted — Latest diff uploaded

Review Board

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.