Add new chains/actors from within chat UI

[JasonWeill]

Problem

Users want to create a custom chain for use with the chat UI. This case already exists in the magic commands; see #46.

Proposed Solution

Add a /register command to the chat UI to register and name a custom chain. This custom chain then becomes available in the settings pane for use as a language model.

Additional context

Because we depend on LangChain, we should use a common interface such as LLMChain for all chains registered in this way.

We will also need an interface for deleting custom chains.