Issue #3009308 by dinazaur, gnuget, MorinLuc0, missvengerberg, tomasbarej, spfaffly, askibinski, vdsh, idebr: Use the official CKEditor mention plugin

This reverts commit 8e5e4553.

Issue #2940516 by gnuget, nedjo: Add an event to allow thirdparty modules do something when a user is mentioned

Added check in hook_editor_settings_alter to account for some formats not having an allowedContent array

Issue #2033739 by askibinski, ruudvanoijen: IE9+ replacement problems when inserting existing strings

Issue #2227509 by 7gipsy: Clicking a proposed username replaces text until next mention or end of paragraph

preventing unnecessary ajax callbacks when typing fast, default = 1 sec.