JI-2406 Fix CKEditor collapsing if calculator element ever is hidden

Overrides the convertToPx function from
https://github.com/ckeditor/ckeditor4/blob/cae20318d46745cc46c811da4e7d68b38ca32449/core/tools.js#L899-L929
and instead adds the calculator element for every measurement
and removes it when it is done with the measurement.