Fixed typo in docstring.
Moved relative URL handling added in refs #21177 to a better place.

Thanks apollo13: "reverse() shouldn't know about absolute/relative paths."
Fixed typo in test docstring.
Fixed #22018 -- Fixed checks for ModelAdmin.fields not handling sub-lists.

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.
Fixed typo in comment.
Fixed two typos.