Publish the size of the book before download

- Closes: #62

lib/template.go

@@ -22,6 +22,7 @@ import (
 var tmpl_funcs = map[string]interface{}{
 	"strings_join": stringsJoin,
 	"download_url": downloadUrl,
+	"size2mb":      size2mb,
 }
 
 type Status struct {
@@ -97,6 +98,10 @@ func downloadUrl(book database.Book) string {
 	return fmt.Sprintf("/download/%s/%s", book.ID, fileName)
 }
 
+func size2mb(size int) float32 {
+	return float32(size) / (1024.0 * 1024.0)
+}
+
 type DevTemplateExecutor struct {
 	assetsPath string
 	tpe        string