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_blob_onebox.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require_relative '../mixins/git_blob_onebox'
+
+module Onebox
+  module Engine
+    class GithubBlobOnebox
+      def self.git_regexp
+        /^https?:\/\/(www\.)?github\.com.*\/blob\//
+      end
+
+      def self.onebox_name
+        "githubblob"
+      end
+
+      include Onebox::Mixins::GitBlobOnebox
+
+      def raw_regexp
+        /github\.com\/(?<user>[^\/]+)\/(?<repo>[^\/]+)\/blob\/(?<sha1>[^\/]+)\/(?<file>[^#]+)(#(L(?<from>[^-]*)(-L(?<to>.*))?))?/mi
+      end
+
+      def raw_template(m)
+        "https://raw.githubusercontent.com/#{m[:user]}/#{m[:repo]}/#{m[:sha1]}/#{m[:file]}"
+      end
+
+      def title
+        Sanitize.fragment(Onebox::Helpers.uri_unencode(link).sub(/^https?\:\/\/github\.com\//, ''))
+      end
+    end
+  end
+end