Custom slash commands in chat UI

[JasonWeill]

Problem

The slash commands provided are hard-coded, and cannot be extended.

Proposed Solution

Let users register new slash commands using classes that extend the BaseChatHandler class. This class should contain a slash_id attribute, which may contain an alphanumeric-and-underscore name. Only one chat handler may declare a particular slash_id.

Additional context

We should use the entry points pattern to allow extension authors to provide additional chat handlers.

This was suggested by @ellisonbg.