Added back `select2('val')`

With the recent changes to how Select2 works internally, this really
isn't needed. This has been added to make the migration path
easier, and it just internally calls `val` on the underlying select
element. The only difference is that the `val` function will now
convert any non-string elements to strings.

The second argument (`triggerChange`) has not been migrated, as
Select2 now internally relies on the `change` event.

**Note:** As the old `initSelection` method has not been migrated,
it is not possible to set the `val` on remote data sources where
the value has not previously been selected.
Added back `select2("enable")`

This also adds a console warning if the `enable` or `val` method is
called.
Fixed backspace on multiple selects
More work on the announcement