Handling env variables during deployment

[ramyaragupathy]

Coming from a discussion on changeset comment #6340:

• there should be logic within code to identify if the default changeset comment var is set in a certain format
• Remove the changeset comment as a default variable. If it is not set, it should be flagged and build fails
• Removing duplicate env variables in the frontend. If they are set in the backend, frontend deployment should automatically access it instead of having to set up manually. At present our deployment covers,
  - Backend via cloudformation and
  - Frontend via yarn build in circle CI and upload to S3 bucket

[ramyaragupathy]

related #6165