Backend message management

The backend needs to receive, parse, and respond to messages. It also needs to package events and deliver them to the front end.

I think there should be a Message Manager in the backend which:

1. Takes a map type and returns a byte array, which [could be used here](https://github.com/emilyagras/agentgopher/blob/master/src/backend/server.go#L36)
1. Parses [the message received from the frontend](https://github.com/emilyagras/agentgopher/blob/master/src/backend/server.go#L35) (potentially as a map) and incorporates the update as appropriate

- [ ] Let's start with a "Clear All" button (https://github.com/emilyagras/agentgopher/issues/23, https://github.com/emilyagras/agentgopher/issues/20)
- [ ] The front end needs to send updates to the backend
- [ ] The back end needs to keep track of what has happened on the front end
- [ ] The clear all button will update both the front end and the back end and set each cell to its default state