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/activitypub/activity/create.rb

@@ -43,7 +43,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
       text: text_from_content || '',
       language: language_from_content,
       spoiler_text: @object['summary'] || '',
-      created_at: @object['published'] || Time.now.utc,
+      created_at: @options[:override_timestamps] ? nil : @object['published'],
       reply: @object['inReplyTo'].present?,
       sensitive: @object['sensitive'] || false,
       visibility: visibility_from_audience,