Document extractor service (#15665)

* Document extractor service

* Fixing vendor modules

* Addressing PR Review comments

* Some small simplifications

* Fixing a linter complain

* simplifying a bit the code using package variables

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

vendor/code.sajari.com/docconv/html_appengine.go

@@ -0,0 +1,18 @@
+// +build appengine
+
+package docconv
+
+import (
+	"io"
+	"io/ioutil"
+	"log"
+)
+
+func HTMLReadability(r io.Reader) []byte {
+	b, err := ioutil.ReadAll(r)
+	if err != nil {
+		log.Printf("HTMLReadability: %v", err)
+		return nil
+	}
+	return b
+}