add subject to atom feed

CHANGELOG.md

config/config.exs

lib/pleroma/web/activity_pub/activity_pub.ex

lib/pleroma/web/feed/feed_controller.ex

lib/pleroma/web/feed/feed_view.ex

...eroma/web/mastodon_api/controllers/timeline_controller.ex

lib/pleroma/web/templates/feed/feed/_activity.xml.eex

lib/pleroma/web/templates/feed/feed/feed.xml.eex

test/web/activity_pub/activity_pub_test.exs

test/web/feed/feed_controller_test.exs