Add extra zeroconf stuff in http to announce mopidy-http as it's own service

This is intended to help clients using the websocket api discover mopidy
Merge branch 'develop' into feature/zeroconf-announce-mopidy
Update default http config to add new zeroconf-websocket option
Update tests to add zeroconf-websocket config option
Merge wlcx/feature/zeroconf-announce-mopidy into develop
http: Polish Mopidy-HTTP Zeroconf service

Builds upon #725 to:

- Remove extra config for Mopidy-HTTP Zeroconf service
- Refactor Zeroconf setup code in the HTTP frontend a bit
- Add documentation