Support listing large numbers of items

[iansan5653]

An error is thrown on parsing JSON from Slack when the List Items value text (encoded JSON) is terminated due to being more than 2,000 characters.

[iansan5653]

Temporarily fixed by disabling item lists for actions with many items.

[iansan5653]

Fixing this will require a significant change in how "List Items" works - the item list will have to be stored by the server, rather than sent with the Slack message.