FEATURE: Allow oneboxing private GitHub URLs (#27705)

This commit adds the ability to onebox private GitHub
commits, pull requests, issues, blobs, and actions using
a new `github_onebox_access_token` site setting. The token
must be set up in correctly to have access to the repos needed.

To do this successfully with the Oneboxer, we need to skip
redirects on the github.com host, otherwise we get a 404
on the URL before it is translated into a GitHub API URL
and has the appropriate headers added.

lib/oneboxer.rb

@@ -681,6 +681,14 @@ module Oneboxer
 
     uri = URI(url)
 
+    # For private GitHub repos, we get a 404 when trying to use
+    # FinalDestination to request the final URL because no auth headers
+    # are sent. In this case we can ignore redirects and go straight to
+    # using Onebox.preview
+    if SiteSetting.github_onebox_access_token.present? && uri.hostname == "github.com"
+      fd_options[:ignore_redirects] << "https://github.com"
+    end
+
     strategy = Oneboxer.ordered_strategies(uri.hostname).shift if strategy.blank?
 
     if strategy && Oneboxer.strategies[strategy][:force_get_host]