Make the streaming API also handle websockets (because trying to get the browser EventSource interface to
work flawlessly was a nightmare). WARNING: This commit makes the web UI connect to the streaming API instead
of ActionCable like before. This means that if you are upgrading, you should set that up beforehand.

redis-backed "mentions" timeline as redundant (given notifications)

https://github.com/alin-rautoiu/mastodroid/issues/23

Add separate sections for Heroku and Vagrant deployments. Add guide for instance administration. Move 'Contribution guide' to 'Development guide'.