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

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

Start by locating the o2_default_post_action_html function and inspect its other call sites, as suggested in the issue. Confirm which Login to Reply links it generates and update the relevant output so those links include rel="nofollow". Done means the repeated Login to Reply links no longer invite search-engine following.

Written by the indexing model from the issue text.