Allow adding arbitrary key value pairs

[rjw57]

Add the ability to add key-value pair information to media items via the API. No not ad search facilities just yet.

Sketch API:

POST/PATCH/PUT: include "custom" object in request

{
    // ...
    "custom": {
        "foo": 57,
    },
}

Do not support anything other than key-value storage. E.g. one could not set:

{
    // ...
    "custom": {
        "foo": { "bar": 57, "buzz": ["a", 3, 4.5] },
    },
}

Required for lecture capture to store and retrieve metadata on media items.