[XB] Thread Reply Whitelist: Invite-Only Thread Replies 2.0.1 Patch Level 2

Fixed thread starters receiving their own invitation alerts

• Fixed incorrect member limit counting when removing users from a whitelist.
• Fixed existing whitelist members being counted again toward the member limit when re-submitted.
• Fixed duplicate invitation alerts being sent to users already on the whitelist.

• Improved caching for large forums
  A new column was added to the xf_thread table to store whitelisted user IDs, enabling faster access checks and reducing unnecessary database queries.
• More control over whitelists
  Configure a maximum number of allowed members to keep whitelists clean.
• Alerts for invitations
  Added notifications when new members are invited, helping users stay informed.

XF 2.3 compatibility

2.0.0 Patch Level 1:
- Fix: "XF::canPerformPreRegAction(): Return value must be of type bool" errors when writing before registering enabled