Upgrade to CKEditor 4.18

Upgrade CKEditor 4.2 installation, that we have to 4.18 release.

How apply upgrade:

1. make sure, that working copy is in SVN 1.8+ format (no ".svn" folder in each sub-folder)
2. delete the "/core/ckeditor/" folder
3. extract attached archive into "/core/" folder (would create "ckeditor" folder)
4. run "svn status | grep '^Unable to render embedded object: File (' ) not found. *//' | xargs -I% svn rm %" command to mark any files deleted in the process as removed in VCS
5. run "svn add . --force" command to add any new files to VCS

P.S.

I've used https://ckeditor.com/cke4/builder with configuration file from "/core/ckeditor/build-config.js" to create upgraded version of CKEditor with our current add-ons.