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

config/application.rb

@@ -9,6 +9,7 @@ Bundler.require(*Rails.groups)
 require_relative '../app/lib/exceptions'
 require_relative '../lib/paperclip/gif_transcoder'
 require_relative '../lib/paperclip/video_transcoder'
+require_relative '../lib/mastodon/snowflake'
 require_relative '../lib/mastodon/version'
 
 Dotenv::Railtie.load