Proposed fix for #18162.
Fixed #19402 - Clarified purpose of CustomUser.REQUIRED_FIELDS

Thanks pydanny for the report and ptone for the patch.
Merge pull request #804 from oinopion/ticket18162

Fixes #18162 --  Specifying choices to Field overrides any form_class argument
Fixed #19282 -- Restored ability to pass Decimals to intcomma filter
Added an example of "default" database dictionary left blank; refs #19775.

Thanks wsmith323 for the patch.
Some style fixes for stuff that was clearly brough tover from doctests.
Fixed #18829 -- Fixed ModelChoiceIterator length

Thanks facundo.olano at gmail.com for the report and thikonom for
the initial patch.