Moved parts of the html separate files

templates/header.html

@@ -0,0 +1,9 @@
+<!doctype html>
+<html lang="en">
+    <head>
+        <title>Shopping List</title>
+         <!-- Latest compiled and minified CSS -->
+        <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
+        <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
+        <link rel="stylesheet" content="text/css" href="/static/css/main.css"/>
+    </head>