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.
git-svn-id: https://develop.svn.wordpress.org/trunk@46564 602fd350-edb4-49c9-b593-d223f7449a82
1f7f0a1357