Add Deploy to Heroku button

[aleccool213]

Closes #72

• Had to add an option for the mongo url so that Heroku-deployed Ackee can just use the default without user configuration
• Adds Deploy to Heroku button
  • I recorded a Loom video showing it off
  • To test this out on this branch, you will need to change the README link to [![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/aleccool213/Ackee/tree/72) so that it uses the correct app.json path

[coveralls]

Pull Request Test Coverage Report for Build 248

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 57.118%

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Totals
Change from base Build 247:	0.0%
Covered Lines:	303
Relevant Lines:	467

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 248

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 57.118%

---

Totals
Change from base Build 247:	0.0%
Covered Lines:	303
Relevant Lines:	467

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💛 - Coveralls

[electerious]

Here we go 😊 9775ec9