Deal with collation-related index corruption (mastodon#14860)

* Add tootctl maintenance fix-duplicates

This tool goes through the database to detect and fix duplicates.
This operation is very slow and may cause data loss (of data that would be
inaccessible without intervention because of the existing index corruptions).
It tries its best to make sensible decisions, and asks the user in some cases.

* Add warning message in db:migrate hook

* Clear Rails cache after being done with database deduplication

Avoids followers hash cache being incorrect, among other things

lib/cli.rb

@@ -13,6 +13,7 @@
 require_relative 'mastodon/cache_cli'
 require_relative 'mastodon/upgrade_cli'
 require_relative 'mastodon/email_domain_blocks_cli'
+require_relative 'mastodon/maintenance_cli'
 require_relative 'mastodon/version'
 
 module Mastodon
@@ -57,6 +58,9 @@ def self.exit_on_failure?
     desc 'email_domain_blocks SUBCOMMAND ...ARGS', 'Manage e-mail domain blocks'
     subcommand 'email_domain_blocks', Mastodon::EmailDomainBlocksCLI
 
+    desc 'maintenance SUBCOMMAND ...ARGS', 'Various maintenance utilities'
+    subcommand 'maintenance', Mastodon::MaintenanceCLI
+
     option :dry_run, type: :boolean
     desc 'self-destruct', 'Erase the server from the federation'
     long_desc <<~LONG_DESC