WIP: Provide the ability to create a split instance in Studio

@andy-armstrong Here is the WIP PR.

In the current implementation, once a split_test instance has been created and a partition_id selected, the select for the partition ID is always disabled (it appears, but you can not change the selection).

To play around with this, set the following in Advanced Settings:

advanced_modules
["split_test"]

user_partitions
[
    {
        "description": "Experiment 1",
        "version": 1,
        "id": 0,
        "groups": [
            {
                "version": 1,
                "id": 0,
                "name": "group 0"
            },
            {
                "version": 1,
                "id": 1,
                "name": "group 1"
            }
        ],
        "name": "Experiment 0,1"
    },
    {
        "description": "Experiment 2",
        "version": 1,
        "id": 1,
        "groups": [
            {
                "version": 1,
                "id": 0,
                "name": "group A"
            },
            {
                "version": 1,
                "id": 1,
                "name": "group B"
            },
            {
                "version": 1,
                "id": 2,
                "name": "group C"
            }
        ],
        "name": "Experiment A,B,C"
    }
]