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)

spec/controllers/api/subscriptions_controller_spec.rb

@@ -45,8 +45,8 @@ RSpec.describe Api::SubscriptionsController, type: :controller do
       post :update, params: { id: account.id }
     end
 
-    it 'returns http created' do
-      expect(response).to have_http_status(:created)
+    it 'returns http success' do
+      expect(response).to have_http_status(:success)
     end
 
     it 'creates statuses for feed' do