Set snowflake IDs for backdated statuses (#5260)

- Rename Mastodon::TimestampIds into Mastodon::Snowflake for clarity
- Skip for statuses coming from inbox, aka delivered in real-time
- Skip for statuses that claim to be from the future

app/lib/ostatus/activity/base.rb

@@ -1,9 +1,10 @@
 # frozen_string_literal: true
 
 class OStatus::Activity::Base
-  def initialize(xml, account = nil)
-    @xml = xml
+  def initialize(xml, account = nil, options = {})
+    @xml     = xml
     @account = account
+    @options = options
   end
 
   def status?