[UX] Add the ckeditor spell-check plugin

I got a request today to add "All the buttons" from CKEditor back into backdrop. When I followed up asking what in particular was missing, one answer I got was the Spell-Check button. I had personally thought this wasn't necessary anymore since browsers and OSes now do spell-check, but let's take a poll. Did anyone use the spell-check feature in Drupal (or in WordPress) and should we include it in Backdrop? Should it be enabled in the editor out-of-the-box?

Existing CKEditor plugins

• https://ckeditor.com/cke4/addon/scayt and https://ckeditor.com/cke4/addon/wsc both use a 3rd party service - so it is a no-no for core.

• https://ckeditor.com/cke4/addon/nanospell also 3rd-party - also no-no

• https://ckeditor.com/cke4/addon/foundeospellchecker

  All spell checking is done on your server without sending content to a third party for checking.

• https://ckeditor.com/cke4/addon/jqueryspellchecker