Custom chat help message

[JasonWeill]

Problem

The chat UI's help message, introduced in #277 to fix #93, is static and cannot be customized by developers or administrators.

Proposed Solution

Allow developers or administrators to customize the chat help message.

Additional context

Should we also allow disabling the help message on startup?

[krassowski]

I think that at minimum the documentation link should be customisable to allow linking to a local copy of the docs.

[JasonWeill]

If #398 is merged in, we should customize the chat help message based on all built-in and custom slash commands.