[UX] CKEditor: Consider adding the Auto Grow and Fixed plugins in core

This was something that was requested in https://github.com/backdrop/backdrop-issues/issues/1087#issuecomment-172...

https://ckeditor.com/cke4/addon/autogrow

With this plugin, CKEditor will automatically expand and shrink vertically depending on the amount and size of content entered in its editing area.

The following configuration options are available:

• maximum and minimum editor height after adjustment to content,
• extra space to be added between content and editor bottom bar,
• having auto grow happen on editor startup