Enable Heroku pipelines

[ephracis]

Move the app into a pipeline and enable Review apps so we can check out the result of each pull request before merging it.

The flow should be like:

Review app -> Staging -> Production

[ephracis]

Oops, closed the wrong one.

[ephracis]

This will cost us money (we have to add more apps, one for each stage). I've created a pipeline and put our current web app at the development stage. When we finally go live we can buy additional apps and put in production (and staging, but those should be temporary). Then we can also enable review apps, but they will cost money as well when they are running.

So until we are getting close to our first release, this one will have to wait.