[FIX] website_blog: Saner crawler instructions

The fixes that this patch includes are:

- The tag URL for tags printed in short and expanded blog posts had no slug, thus producing a brand new URL. Now they have the slug.
- Any request landed without that slug is slugified with a 301 code, as with the rest of Odoo.
- The `<a>` elements in right column archives have now `rel="nofollow"`.
- The `<a>` elements in right column tag cloud, where more than 1 tag is used, have now `rel="nofollow"`.

closes odoo/odoo#30832

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>