"Login to Reply" buttons should have a "nofollow" rel attribute

[braders]

The "Login to Reply" buttons are repeated frequently and link to a page of low SEO value. As such, they should not be followed by search engines.

It looks like this change needs to happen in the o2_default_post_action_html function (other uses of the o2_default_post_action_html function seem to also be safe to apply nofollow to).

This was originally raised in the context of the wordpress.org sites: https://meta.trac.wordpress.org
/ticket/4808