Select2 integration for pickers

This PR adds select2 integration for the pickers on the client side.

TODO:

- [x] Add data providers for the pickers
- [x] Update tests for `ExactMatchFilter`, `SearchFilter` and `CallbackFilter` so that they support the new filter join types added in `FilterCollection`
- [x] Add routes for serving remote data to pickers
- [x] Create mixin for select2 initialisation for use by View objects
- [x] Fix issue with `FilterCollectionBuilder` not building from request data properly
- [x] Add support for matching multiple values in `ExactMatchFilter` 
- [x] Integrate select2 assets
- [x] Integrate select2 custom CSS implemented by @StuartRayson