Draft: Show the effective values in the preferences view.

Here is a preview of how it looks. Right now, I've only done this for address based preferences, but it can be done for membership based preferences too. A lot of legwork is done as a part of _combined_preferences() method in each of the views, it just needs to be plumbed through.

If the field's value is empty, it will show the "Value" as the "effective" value as Mailman Core would calculate it.

Fixes #476