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>

spec/lib/onebox/engine/html_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe Onebox::Engine::HTML do
+  before do
+    @link = "http://amazon.com"
+
+    stub_request(:get, @link).to_return(status: 200, body: onebox_response("amazon"))
+    stub_request(:get, "https://www.amazon.com/Seven-Languages-Weeks-Programming-Programmers/dp/193435659X")
+      .to_return(status: 200, body: onebox_response("amazon"))
+  end
+
+  describe "#raw" do
+    class OneboxEngineHTML
+      include Onebox::Engine
+      include Onebox::Engine::HTML
+
+      def initialize(link)
+        @url = link
+        @options = {}
+      end
+    end
+
+    it "returns a Nokogiri object that has a css method" do
+      object = OneboxEngineHTML.new("http://amazon.com").send(:raw)
+      expect(object).to respond_to(:css)
+    end
+  end
+end