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/creation.rb

@@ -34,7 +34,7 @@ class OStatus::Activity::Creation < OStatus::Activity::Base
         reblog: cached_reblog,
         text: content,
         spoiler_text: content_warning,
-        created_at: published,
+        created_at: @options[:override_timestamps] ? nil : published,
         reply: thread?,
         language: content_language,
         visibility: visibility_scope,