Use preview deployments

A staging server would make it easier to -

1. Make frontend contribs by cloning and running only the Rocketmeet-client repo.
2. Test new or revised API endpoints across machines before they are pushed to prod.

[anandbaburajan]

Thanks @joelerego for the suggestion! As discussed at Gitter, we'll look into this once we upgrade our Heroku plan.

[anandbaburajan]

Update: to achieve this, we'll contact Vercel for their OSS sponsorship so that we can get a free Team plan.