Set cleanup_attachments to true by default

Since bcfbfbcff5 (part of
<#789>), the overhead for
cleaning up orphaned attachments has been drattically reduced.

Most admins are unaware of this option even existing, but may notice an
increase in the size of the uploads directory (or S3 bucket size if used
instead) even if auto-expiring posts are used. This should hopefully
make this problem more manageable.

For those that still encounter performance issues, the setting can still
be disabled if needed.

config/config.exs

@@ -251,7 +251,7 @@
   registration_reason_length: 500,
   external_user_synchronization: true,
   extended_nickname_format: true,
-  cleanup_attachments: false,
+  cleanup_attachments: true,
   cleanup_attachments_delay: 1800,
   multi_factor_authentication: [
     totp: [