Gift Upgrades 2.6.0

• Allow users with permission to gift user upgrades for free.
• When fresh installing the add-on, ensure the registered group gets permissions to send gifts.
• Add a gift link to messages sent within Siropu's Chat add-on.
• Adjust permission checking when gifting content.
• Minor code clean up.

Includes changes from v2.3.0 because I forgot to post that, oops

• Add option "Exclude warned posts" from recent gifted posts.
• Add an icon to the "Gift" button.
• Fix MySQL 8.0 compatability when upgrading from versions earlier than 2.1.0.

• Now requires PHP 7.0+.
• Now depends on Standard Library by Xon.
• Properly hide "Gift" button for guests.

• Improve compatibility with a private add-on.

• Add content gifting support for Calendar by NixFifty.
• Add content gifting support for Article Management System by @Bob.
• Minor phrasing tweaks.
• Fix gifting user upgrade purchased by the gifter.
• Add UI for gifting purchased upgrades from the Account Upgrades page.

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.