xF2 Add-on Custom Fields Permissions 2.7.1

Ensure displaying of custom fields fails closed during add-on update.

• During future add-on updates, all custom fields will stop displaying for the duration of the update. Designed to ensure custom fields do not leak information
• Disabling the add-on will render all custom fields visible!

• Workaround that on upgrading XenForo, custom field caches will be rebuild without extra information. Thanks @DragonByte Tech

• Rework to be more extendable
• Support content user permission checks (Threads)

• Now supports thread custom fields. Thanks @DragonByte Tech!

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

• Fix errors when adding/editing thread custom fields