Switch server from unicorn to puma

[nimmolo]

This PR is to help switch the production server from unicorn to puma, without turning on threading.

---

It follows the only complete guide I've found, but it's pretty old:
Migrating from unicorn to puma with no downtime

Current preferred way is to run Puma via systemd, not init.d
Note that there is no file /etc/systemd/system/unicorn.service on production, what we have is /etc/init.d/unicorn

• Add gem puma in this branch
• Configure /config/puma.rb in this branch
• Configure /config/puma-dev.rb in this branch (in case Jason wants to run it locally)
• Configure systemd service /config/etc/puma.service and copy to /etc/systemd/system/puma.service on production
• Alter our /deploy.sh in this branch not to reload Unicorn
• Modify config/etc/nginx.confin this branch to point to new Puma socket instead of Unicorn
  Check example of upstream and proxy_pass configs here

  upstream puma {
    # fail_timeout=0 means we always retry an upstream even if it failed
    # to return a good HTTP response (in case the Puma master nukes a
    # single worker for timing out)
    server unix:/var/web/mo/tmp/sockets/puma.sock fail_timeout=0;
  }

# also remove or fix proxy_pass: 

    location @app {
      ...
      # this is where puma is listening
      proxy_pass http://puma;

Deploy this PR. Then:

• Copy /config/etc/nginx.conf to /etc/nginx/nginx.conf
• sudo systemctl start puma.service so it's running alongside Unicorn
• sudo systemctl status puma.service be sure it's running
• sudo systemctl reload nginx (to listen to the Puma socket)
• sudo systemctl stop unicorn.service (or maybe sudo uni stop, since there's no such thing as unicorn.service)
• sudo systemctl disable unicorn.service (or maybe remove the init file in /etc/init.d/unicorn)
• sudo systemctl enable puma.service

---

Reference:
Benefits of Switching from Unicorn to Puma
Puma docs: Configuration and Deployment
How Do I Know Whether My Rails App Is Thread-safe or Not?
Do we want to consider using Capistrano for deployment?

[nimmolo]

Step 1: Improve thread safety. PR #1932

[coveralls]

coverage: 94.532%. remained the same
when pulling cf37548 on unicorn-to-puma
into 3286363 on main.

[mo-nathan]

Was just thinking about this due to the outage this morning. Great to see progress here!