Update CKEditor library to 4.10.0

Update from 4.9.2 (which was also added to D8.6 only recently: #2962330: Update CKEditor library to 4.9.2) to CKEditor 4.9.2: https://ckeditor.com/blog/CKEditor-4.10-released/

4.10 contains new features through new plugins (mentions, tags, emoji, autocomplete), but we're not adding those here; this issue is solely about updating our CKEditor build to the latest and greatest, without changing the set of plugins we use.

4.10 also contains some significant bugfixes: https://ckeditor.com/blog/CKEditor-4.10-released/#bug-fixes, including improvements for screen readers!

See also:

#1858210: [meta] Content editing experience follow-ups — in-place editing and WYSIWYG
#1950098: Update CKEditor library to 4.1
#2036253: Update CKEditor library to 4.2
#2039163: Update CKEditor library to 4.4
#2271051: Update CKEditor library to 4.4.4
#2345961: Update CKEditor library to 4.4.5
#2384581: Security: Update CKEditor library to 4.4.6
#2415111: Update CKEditor library to 4.4.7
#2521820: Update CKEditor library to 4.5.3
#2321583: Update CKEditor library to 4.5.5
#2663566: Update CKEditor library to 4.5.7
#2698587: Update CKEditor library to 4.5.8
#2724225: Update CKEditor library to 4.5.9
#2765751: Update CKEditor library to 4.5.10
#2797427: Update CKEditor library to 4.5.11
#2828494: Update CKEditor library to 4.6
#2848215: Update CKEditor library to 4.6.2
#2893566: Update CKEditor library to 4.7.1
#2904142: Update CKEditor library to 4.7.2
#2908864: Update CKEditor library to 4.7.3
#2926932: Update CKEditor library to 4.8.0
#2962330: Update CKEditor library to 4.9.2