wordpress: Add .well-known/webfinger redirect for ActivityPub plugin

Since WordPress is served at /wordpress path, Webfinger queries from other ActivityPub servers won't reach it.

The ActivityPub plugin page describes adding a redirect for this case: https://wordpress.org/plugins/activitypub/#what%20if%20you%20are%20running%20your%20blog%20in%20a%20subdirectory%3F

We will want to be more specific and only redirect for webfinger. (I assume WordPress is the only application we have using Webfinger currently.)