[#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

app/mailers/assignment_mailer.rb

@@ -3,6 +3,8 @@ class AssignmentMailer < ApplicationMailer
   layout 'mailer'
 
   def conversation_assigned(conversation, agent)
+    return if ENV.fetch('SMTP_ADDRESS', nil).blank?
+
     @agent = agent
     @conversation = conversation
     mail(to: @agent.email, subject: "#{@agent.name}, A new conversation [ID - #{@conversation.display_id}] has been assigned to you.")