Entity picker abstraction and refactoring

This PR is a divergence from the **feature/client-bundle** branch, and contains updates to the entity pickers to allow the creation of the client picker in the ClientBundle.

When this branch is merged, #42 can then be completed.

TODO:

- [x] Add data transformers for pickers 
- [x] Refactor pickers to extend from `AbstractPickerType`
- [x] Remove `UserPickerTypeTest` and move into `AbstractPickerTypeTest`
- [x] Remove unnecessary tests for concrete picker implementations
- [x] Add tests for concrete data transformers
- [x] Add picker type to `ProjectFormType` and update tests
- [x] Add support for `AbstractPickerType::RESTRICTION_SINGLE` in the data transformers (should transform to and from a single instance always)