Composable verbose names

[dokterbob]

Allow using parent names to dynamically compose verbose names of subfields, like so:

    class IntegerEstimatedRange(CompositeField):
        minimum = models.DurationField(
            lambda n: _("%(parent_verbose_name)s minimum") % {
                "parent_verbose_name": n
            }
        )

    class Species(models.Model):
        height = IntegerEstimatedRange(verbose_name=_("plant height"))

See proposed documentation. Test added for it as well.

[bikeshedder]

Sorry for taking so long. I almost forgot about this issue.

In the example of this issue you've used a lambda expression but the code just relies on formatting via the % operator.

I guess passing a lambda as verbose_name didn't work?