R1:ca0115b361a3

Support configuration-driven custom fields

Summary:
Ref T1702. Ref T3718. There are a couple of things going on here:

**PhabricatorCustomFieldList**: I added `PhabricatorCustomFieldList`, which is just a convenience class for dealing with lists of fields. Often, current field code does something like this inline in a Controller:

foreach ($fields as $field) {
// do some junk
}

Often, that junk has some slightly subtle implications. Move all of it to `$list->doSomeJunk()` methods (like…