Updating Pickers to support select2

Adds support for select2 integration with the picker types.

Previously, we had a very complicated approach to supporting pickers in the view (we built `displayValues` view property for containing selection display values), and had to rely on both decorators and transformers.

This has now been simplified, so we only depend on transformers in the pickers. The pickers will:

- reverse transform data from scalar (e.g. "1, 2, 3") into objects (e.g. entities with IDs 1, 2 and 3)
- transform data from objects into JSON representation (select2 support, e.g. `[{id:1,text:'Mark Wilson'}]"`)

TODO: 

- [x] Update the picker transformers so that they return JSON representation of objects in `transform()` method