Support custom fields without corresponding model attributes

[shouichi]

The current implementation assumes fields have corresponding model
attributes. But sometimes, we just want to display a button or a link
that is not backed by an attribute.

This change simply relaxes the assumption and allows custom fields that
are not tied to model attributes.

See also #1468.

[nickcharlton]

Thanks for opening this!

How does this compare to #2658?

[shouichi]

This PR is simpler than #2658 (only one line change in lib and other changes are just for tests). It just relaxes the assumption, no feature was added. Each application can implement its virtual attributes and administrate doesn't need to know anything.

[goosys]

I tried creating a similar NonAttributeField in the #2658 branch, and it worked perfectly.
Unless there’s a specific intention to raise an exception when accessing an attribute that doesn’t exist in the model, I believe this approach is sufficient.

[nickcharlton]

Thanks, that's helpful.

I think I conceptually prefer this, as the code change is much smaller compared to the other. But I need to think about it!