DEV: Absorb onebox gem into core (#12979)

* Move onebox gem in core library

* Update template file path

* Remove warning for onebox gem caching

* Remove onebox version file

* Remove onebox gem

* Add sanitize gem

* Require onebox library in lazy-yt plugin

* Remove onebox web specific code

This code was used in standalone onebox Sinatra application

* Merge Discourse specific AllowlistedGenericOnebox engine in core

* Fix onebox engine filenames to match class name casing

* Move onebox specs from gem into core

* DEV: Rename `response` helper to `onebox_response`

Fixes a naming collision.

* Require rails_helper

* Don't use `before/after(:all)`

* Whitespace

* Remove fakeweb

* Remove poor unit tests

* DEV: Re-add fakeweb, plugins are using it

* Move onebox helpers

* Stub Instagram API

* FIX: Follow additional redirect status codes (#476)

Don’t throw errors if we encounter 303, 307 or 308 HTTP status codes in responses

* Remove an empty file

* DEV: Update the license file

Using the copy from https://choosealicense.com/licenses/gpl-2.0/#

Hopefully this will enable GitHub to show the license UI?

* DEV: Update embedded copyrights

* DEV: Add Onebox copyright notice

* DEV: Add MIT license, convert COPYRIGHT.txt to md

* DEV: Remove an incorrect copyright claim

Co-authored-by: Jarek Radosz <jradosz@gmail.com>
Co-authored-by: jbrw <jamie@goatforce5.org>

lib/onebox/engine/github_pull_request_onebox.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require_relative '../mixins/github_body'
+
+module Onebox
+  module Engine
+    class GithubPullRequestOnebox
+      include Engine
+      include LayoutSupport
+      include JSON
+      include Onebox::Mixins::GithubBody
+
+      GITHUB_COMMENT_REGEX = /(<!--.*?-->\r\n)/
+
+      matches_regexp(/^https?:\/\/(?:www\.)?(?:(?:\w)+\.)?(github)\.com(?:\/)?(?:.)*\/pull/)
+      always_https
+
+      def url
+        "https://api.github.com/repos/#{match[:owner]}/#{match[:repository]}/pulls/#{match[:number]}"
+      end
+
+      private
+
+      def match
+        @match ||= @url.match(%r{github\.com/(?<owner>[^/]+)/(?<repository>[^/]+)/pull/(?<number>[^/]+)})
+      end
+
+      def data
+        result = raw.clone
+        result['link'] = link
+
+        created_at = Time.parse(result['created_at'])
+        result['created_at'] = created_at.strftime("%I:%M%p - %d %b %y %Z")
+        result['created_at_date'] = created_at.strftime("%F")
+        result['created_at_time'] = created_at.strftime("%T")
+
+        ulink = URI(link)
+        result['domain'] = "#{ulink.host}/#{ulink.path.split('/')[1]}/#{ulink.path.split('/')[2]}"
+
+        result['body'], result['excerpt'] = compute_body(result['body'])
+
+        result
+      end
+    end
+  end
+end