Fix bugs which OStatus accounts may detected as ActivityPub ready (#4662)

* Fallback to OStatus in FetchAtomService

* Skip activity+json link if that activity is Person without inbox
* If unsupported activity was detected and all other URLs failed, retry with ActivityPub-less Accept header

* Allow mention to OStatus account in ActivityPub

* Don't update profile with inbox-less Person object

app/services/activitypub/process_account_service.rb

@@ -6,6 +6,8 @@ class ActivityPub::ProcessAccountService < BaseService
   # Should be called with confirmed valid JSON
   # and WebFinger-resolved username and domain
   def call(username, domain, json)
+    return unless json['inbox'].present?
+
     @json     = json
     @uri      = @json['id']
     @username = username