Comments: Add `rel="nofollow ugc"` attribute when converting plain URLs to...

Comments: Add `rel="nofollow ugc"` attribute when converting plain URLs to `<a>` tags in comments via `make_clickable()`.

Introduce `make_clickable_rel` filter for the `rel` value that is added to URL matches converted to links.

This is a follow-up to [46349], which added the `rel="nofollow ugc"` attribute to existing `<a>` tags in comments via `wp_rel_ugc()`.

UGC stands for User Generated Content, and the `ugc` attribute value is recommended for links within user generated content, such as comments and forum posts.

See https://webmasters.googleblog.com/2019/09/evolving-nofollow-new-ways-to-identify.html.

Props blogginglife, SergeyBiryukov.
Reviewed by desrosj, audrasjb.
Fixes #48022.
Built from https://develop.svn.wordpress.org/trunk@46564


git-svn-id: https://core.svn.wordpress.org/trunk@46361 1a063a9b-81f0-0310-95a4-ce76da25c4cd