feat(cms): add noindex for bots (#1881)

Adding the noindex tag can prevent Google from indexing the CMS admin page.

netlify-cms-academic/static/admin/index.html

@@ -1,6 +1,7 @@
 <!doctype html>
 <html lang="en-us">
 <head>
+  <meta name="robots" content="noindex">
   <meta charset="utf-8">
   <meta content="width=device-width, initial-scale=1.0" name="viewport">
   <title>Wowchemy Content Manager</title>