title: Implement OAuth provider in Rails for use with a single client application
body: <p>I'm working on building a closed community forum for members of an existing site I also built. I'd like to allow members to sign in to the new forum using their existing accounts on the main application. The forum will be on a different domain from the main site.</p>
  
  <p>This will be my first time using OAuth. Most information on the web about  it seems to skew heavily toward allowing users to sign in with Twitter/Facebook/etc accounts, and there's very little information that seems pertinent to what I'm working on.</p>
  
  <p>I'd like to create an OAuth provider in the original web app and a OAuth client in the new app I'm building. No other providers will be used, and no other client sites will be authenticating off this main site. Additionally, I need to avoid any sort of "authorize this web site" intermediary step for my users.</p>
  
  <p>The main site is Rails 4 with Devise, and the new site is Rails 5. I'd like to stick with Devise on the new site as well, which I understand can function as an OAuth client. Doorkeeper seems like it could be overkill for my exceedingly simple needs, but most other information I've found is years out of date, so I don't want to invest time pursuing an implementation that won't work or is not the current best approach.</p>
  
  <p>What is currently the best way to handle this kind of single-client, single-provider setup?</p>