Retrieve avatar via ActivityPub

[akirk]

Fixes #139.

This will use an avatar from ActivityPub when available:

You can also update an avatar from ActivityPub (as it will not (yet?) update the avatar automatically when a user changes it):

[gbraad]

Wanted to try this out, but got an error:

[Thu Dec 29 20:37:59.123047 2022] [fcgid:warn] [pid 340940:tid 3637677836032] [client 45.158.40.164:50940] mod_fcgid: stderr: PHP Fatal error:  Declaration of Activitypub\\Friends_Feed_Parser_ActivityPub::fetch_feed($url) must be compatible with Friends\\Feed_Parser::fetch_feed($url, ?Friends\\User_Feed $user_feed = null) in /home/spotsnel_gbraad/sites/gbraad.nl/blog/wp-content/plugins/activitypub/integration/class-friends-feed-parser-activitypub.php on line 137

[akirk]

@gbraad thanks for flagging, I tried to fix it in #143.