xF2 Add-on Change Post Date by Xon 2.2.0

• Require XenForo 2.2+
• Require php 7+
• Fix internal server error instead of a human-readable error message when an unparsable date format was given.
  • Error message now recommends ISO 8601, eg 2000-12-31 23:55
• Rebuild forum's last post data if applicable

• Minor cleanup & slightly better error handling

• Tweak execution order of class extensions/template modifications/etc to provide predictable ordering
• Add LICENCE and README.MD to redistributable.
• Remove non-functional template modification

• Implement using service object for better extendability.
  • Required for Sticky Any Post for XF2 support
• Add moderator event logging