Move special-case logic in BoundField.initial() to BaseForm.get_initial_for_field()

[Chris Jerdonek]

This is another follow-up to ticket #32920.

Currently, BoundField.initial() has logic to special-case time and datetime objects:
​https://github.com/django/django/blob/f5669fd7b568cf8a3eda1e65c1c6fb583c7b177d/django/forms/boundfield.py#L217-L219
I noticed that this logic can be moved to BaseForm.get_initial_for_field(), and in particular under the if callable(value) block:
​https://github.com/django/django/blob/f5669fd7b568cf8a3eda1e65c1c6fb583c7b177d/django/forms/forms.py#L496-L497

Eventually, I think it could make sense to go further and move some of this logic to a new method of the Field class, which could permit the special-casing to be handled by overriding in sub-classes that use times and datetimes.

[Chris Jerdonek]

PR: ​https://github.com/django/django/pull/14641

[Carlton Gibson <carlton.gibson@…>]

In 469e769:

Refs #32924 -- Changed test_get_initial_for_field() to use subTests.

[Carlton Gibson <carlton.gibson@…>]

In 788441c:

Refs #32924 -- Changed test_initial_datetime_values() to use subTests.

[Carlton Gibson <carlton.gibson@…>]

In 0dc25526:

Fixed #32924 -- Changed BaseForm.get_initial_for_field() to remove microseconds when needed.