Add plugin widget

Change-Id: I55f006bfe1d60e6fe8e73b801171e507a62b97d9
diff --git a/service.go b/service.go
index ba009b8..5d82e15 100644
--- a/service.go
+++ b/service.go
@@ -68,7 +68,17 @@
 
 func setupRouter() *gin.Engine {
 	r := gin.Default()
+	r.LoadHTMLGlob("templates/*")
+
+	r.GET("/index", func(c *gin.Context) {
+		c.HTML(http.StatusOK, "main.html", gin.H{
+			"title":       "External Sale Plugin",
+			"korapServer": "https://korap.ids-mannheim.de/",
+		})
+	})
+
 	r.GET("/:corpus_id/:doc_id/:text_id", CheckSaleUrl)
+	r.Static("/assets", "./assets")
 	return r
 }