Fixed bug that occured when setting up review.ninja from scratch

The ```redirect_uri``` GET query parameter for the GitHub did not contain the port of the application, since the environment variable ```HOST_PORT``` was undefined. 

I got this: ```The+redirect_uri+MUST+match+the+registered+callback+URL+for+this+application.``` 

Review.Ninja documentation tells to use ```PORT``` and for all the cloud hosters ```PORT``` is also the convention.

@dtornow @dfarr @flower-pot Please double-check if this is the correct way you intended.