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/slides_onebox.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module Onebox
+  module Engine
+    class SlidesOnebox
+      include Engine
+      include StandardEmbed
+
+      matches_regexp(/^https?:\/\/slides\.com\/[\p{Alnum}_\-]+\/[\p{Alnum}_\-]+$/)
+      requires_iframe_origins "https://slides.com"
+
+      def to_html
+        <<-HTML
+          <iframe
+            src="https://slides.com#{uri.path}/embed?style=light"
+            width="576"
+            height="420"
+            scrolling="no"
+            frameborder="0"
+            webkitallowfullscreen
+            mozallowfullscreen
+            allowfullscreen
+          ></iframe>
+        HTML
+      end
+
+      def placeholder_html
+        escaped_src = ::Onebox::Helpers.normalize_url_for_output(raw[:image])
+        "<img src='#{escaped_src}'>"
+      end
+    end
+  end
+end