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/slides_onebox_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe Onebox::Engine::SlidesOnebox do
+  let(:link) { "http://slides.com/drksephy/ecmascript-2015" }
+  let(:html) { described_class.new(link).to_html }
+
+  before do
+    stub_request(:get, link).to_return(status: 200, body: onebox_response("slides"))
+  end
+
+  describe "#placeholder_html" do
+    it "returns an image as the placeholder" do
+      expect(Onebox.preview(link).placeholder_html).to include("//s3.amazonaws.com/media-p.slid.es/thumbnails/secure/cff7c3/decks.jpg")
+    end
+  end
+
+  describe "#to_html" do
+    it "returns iframe embed" do
+      expect(html).to include(URI(link).path)
+      expect(html).to include("iframe")
+    end
+  end
+end