Extract PUSHER_URL out of NotificationsController and into environment variable

[ybakos]

Use the "dotenv" gem.
Create a .env file - this should not be in the repo.
Configure heroku staging and production, eg heroku config:add PUSHER_URL="https://..." -rproduction

Lastly, copy .env to .env.example that looks like:

PUSHER_URL="https://example.com"