edited the heroku section

[smcalilly]

Overview

This PR is for #161. I've made changes to the Heroku deployment instructions, based on the discussions in that issue.

• edited the Heroku CLI steps
• reorganized Heroku config info
• added a troubleshooting section

Notes

Please don't hesitate to edit for clarity. Also, scrutinize my terminology. I'm still not 100% certain about the distinction between "apps" and "pipelines".

The diff doesn't show how/where I moved the configuration steps, but they were moved into the "Set up application code for Heroku" section.

Testing Instructions

• Make sure the heading links still work and point to the correct place, because I changed one and moved another.
• Make sure the new link in the troubleshooting section works.
• Make sure it makes sense.
• Make sure it fits the style and tone.
• Make sure it's helpful.

[smcalilly]

@hancush I've made your requested changes. Let me know if I missed anything (collaborating on writing via a GitHub PR isn't the best UX)