Gift Upgrades 2.4.3

Thanks to @Xon for contributions to this release.

• Re-introduce content gifting. Posts, profile posts, profile post comments. Last 2 require XF2.1.
• Add missing legacy gift_callback.php endpoint for XF1.x migrations.
• Add recently gifted post's widget (can migrate from XF1.x config).
• Fixed alerts about received gifts.
• Re-add user criteria.
• Supports User Essentials "Custom User Upgrade Amounts" for gifts.
• Supports Expiring User Upgrades to prevent sending duplicate notifications.

• Remove unused options.
• Fix potential conflict when trying to gift an upgrade that you already have.