Allow services to emulate/impersonate user (send with username)

It would be great if the API allowed bots to specify a username with each request. This would make it possible to write a bridge to a Matrix or IRC channel, for instance, with matching usernames.

Related: #662

Previous implementation plan

Iteration 1

• Add botInternalUserId field to the chat message schema
  • Allow this to be passed in from the API
• Update the duplicate-chat-detector.js to sum on botInternalUserId if available
• Update the chat report service to sum on botInternalUserId if available

Iteration 2

• Add in rich avatar and display name

Implementation

Add a message.virtualUser field to the chat message schema.

"virtualUser": {
	"type": "matrix",
	"externalId": "madlittlemods:matrix.org",
	"displayName": "madlittlemods (Eric Eastwood)",
	"avatarUrl": "https://matrix-client.matrix.org/_matrix/media/r0/thumbnail/matrix.org/bDayqThxTIcGNcskzIADknRv?width=30&height=30&method=crop"
},

• We may need to stub message.fromUser to support the legacy clients like the Android/iOS easier.
• Support room bans for virtual users
• Support chat spam bans for virtual users