- simplify initialization procedure
- set user display name as the Etherpad name\
- use SharedDocumentButton

Use subscribers to detect state change and emit those
out to other participants. Use middleware to register
the command listener.

Using anything non-serializable for action types is discouraged:
https://redux.js.org/faq/actions#actions

In fact, this is the Flow definition for dispatching actions:

declare export type DispatchAPI<A> = (action: A) => A;
declare export type Dispatch<A: { type: $Subtype<string> }> = DispatchAPI<A>;

Note how the `type` field is defined as a subtype of string, which Symbol isn’t.