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/status_check.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module Onebox
+  class StatusCheck
+    def initialize(url, options = Onebox.options)
+      @url = url
+      @options = options
+      @status = -1
+    end
+
+    def ok?
+      status > 199 && status < 300
+    end
+
+    def status
+      check if @status == -1
+      @status
+    end
+
+    def human_status
+      case status
+      when 0
+        :connection_error
+      when 200..299
+        :success
+      when 400..499
+        :client_error
+      when 500..599
+        :server_error
+      else
+        :unknown_error
+      end
+    end
+
+    private
+
+    def check
+      res = URI.open(@url, read_timeout: (@options.timeout || Onebox.options.timeout))
+      @status = res.status.first.to_i
+    rescue OpenURI::HTTPError => e
+      @status = e.io.status.first.to_i
+    rescue Timeout::Error, Errno::ECONNREFUSED, Net::HTTPError
+      @status = 0
+    end
+  end
+end