[#260] One click deploy to heroku (#275)

* [#260] One click deploy to heroku

* Added app.json file for Heroku deployment
* Made changes in Procfile to accept the PORT as env variable
* Added the one click button in README.md

* Change readme and link

* Alignment of button in Readme

* Changing redis to free plan

* Removed node-js build-pack

* Changed the post-deploy script to be rake db:migrate

* Removed web concurrency variable from app.json for heroku

* Changed the link to chatwoot logo referenced in app.json

* Changed postdeploy hook

* Changed logo to be raw content from github

* Changed the SMTP variables

* Added optional conditional for sending mail
* Changed the naming of SMTP variables

* Having logo as base 64 encoded image for heroku deploy page

* Fixed key not found error for SMTP variables

* Correcting the specs for conversation assignment mailer

* Spec rubocop fixes

* Spec rubocop fixes
* Added the link to master for heroku app.json

README.md

@@ -30,6 +30,12 @@ Detailed documentation is available at [www.chatwoot.com/docs](https://www.chatw
 
 You can find the quick setup docs [here](https://www.chatwoot.com/docs/quick-setup).
 
+## Heroku one-click deploy
+
+Deploying chatwoot to heroku, it's a breeze. It's as simple as clicking this button.
+
+[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/chatwoot/chatwoot/tree/master)
+
 ## Contributors ✨
 
 Thanks goes to all these [wonderful people](https://www.chatwoot.com/docs/contributors):