Flatten a level of sublists before checking for duplicate fields.
When given sublists such as:
```python
class FooAdmin(admin.ModelAdmin):
fields = ('one', ('one', 'two'))
```
The previous code did not correctly detect the duplicated 'one' field.
Thanks to jwa for the report.
During the admin check for list_editable _check_list_editable_item
should return an empty list if all checks pass. Additionally the
Testcase test_readonly_and_editable was changed to test what the
name implies instead of duplicating the logic of test_readonly.
This is the result of Christopher Medrela's 2013 Summer of Code project.
Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.
Also: Fixes #8579, fixes #3055, fixes #19844.
