Remove ngrok dependency

[dibyadas]

As you can see, the 1px image that is added to the mail is provided by a server hosted locally and exposed by ngrok so that gmail servers can access it. This was done for development locally. Instead it is better to host it in heroku and use that. This way, the test will be more robust and the problem of cache will hopefully not remain.
In short, host the app.py in heroku and put that link in the content.js file.