[FIX] website_blog: prevent crawler on multiple blog tags page

Before this commit:
If a blog had multiple tags on the page, the crawler would click on every one
of them one after another until every tags combination got crawled.
With a lot of tags, this could result in million of URLs.

Now, we prevent URLs with more than one tag to be crawled.

Eg: With tags 'A', 'B', C':
/A
/A/B
/A/C
/A/B/C
/A/C/B

/B
/B/A
/B/C
/B/A/C
/B/C/A

/C
/C/A
/C/B
/C/A/B
/C/B/A

addons/website_blog/views/website_blog_templates.xml

@@ -103,6 +103,7 @@
     <t t-call="website_blog.index">
         <t t-set="head">
             <link t-att-href="'/blog/%s/feed' % (blog.id)" type="application/atom+xml" rel="alternate" title="Atom Feed"/>
+            <meta t-if="len(active_tag_ids) > 1" name="robots" content="noindex, nofollow"/>
         </t>
         <div class="container">
             <t t-call="website.pager" >