User Mentions Improvements by Xon 2.10.2

Fix "Call to a member function offsetExists() on null" that could occur with some user custom fields configurations

• Fix quoted email phrase
• Improve compatibility of usergroup mention tagging between XF versions (Fix XF2.2.0B2 support)

• Fix possible error for guest posts with user mentions

• Requires php 7.0+
• Now depends on Standard Library by Xon
• Supports XF2.2

• Fix incorrect access level when extending user changelog

• Fix user change log showing raw column names added by this add-on

• Fix typo in quoted email phrase
• Fix incorrect phrase usage for private groups
• Fix incorrect view permission check on private groups resulting in usergroup membership disclosure if the "[UMI] View mentionable user group members" permission had been granted and the user-group was mentionable.

• Fix typo blocking user mention emailing

• Ensure duplication mention emails for reports are not sent (requires Report Mention Improvements v2.4.0+)
• Fix user mention emails are not reliably sent

• Fix "Call to a member function isValidRelation() on null"