Default (local-echo) display name for chatterbox users

[rxl881]

Currently, chatterbox displays fairly long and unweildy user IDs for new users by default (as shown in the screenshot below). It would be good to be able to set a default value to be shown as the display name of the chatterbox user in the local client if one has not been specified (e.g. displayname === undefined or displayname === matrixUserId).

Ideally this would simply be a local representation of the display name in the client, rather than setting the Matrix display name for the chatterbox users, as this would result in all chatterbox users appearing the same in full Matrix clients like Element-web, and could cause confusion, for example in the Element-web left panel.

For now it would be good for the default, local display name for chatterbox users to be "Me". However, it would be good for this to be easily changable in futture, as we may wish to use other values like "Guest" etc., instead.