Fix #2706 - Always respond with 200 to PuSH payloads (#2733)

Fix #2196 - Respond with 201 when Salmon accepted, 400 when unverified
Fix #2629 - Correctly handle confirm_domain? for local accounts
Unify rules for extracting author acct from XML, prefer <email>, fall back
to <name> + <uri> (see also #2017, #2172)

app/services/concerns/author_extractor.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module AuthorExtractor
+  def author_from_xml(xml)
+    # Try <email> for acct
+    acct = xml.at_xpath('./xmlns:author/xmlns:email', xmlns: TagManager::XMLNS)&.content
+
+    # Try <name> + <uri>
+    if acct.blank?
+      username = xml.at_xpath('./xmlns:author/xmlns:name', xmlns: TagManager::XMLNS)&.content
+      uri      = xml.at_xpath('./xmlns:author/xmlns:uri', xmlns: TagManager::XMLNS)&.content
+
+      return nil if username.blank? || uri.blank?
+
+      domain = Addressable::URI.parse(uri).normalize.host
+      acct   = "#{username}@#{domain}"
+    end
+
+    FollowRemoteAccountService.new.call(acct)
+  end
+end