Workflowy: disable spellcheck von Tiesmaster

This disables spellchecking for the web application Workflowy, as this is not possible to disable manually in the webapplication itself. This applies the superuser.com approach (https://superuser.com/questions/1284546/how-to-disable-spell-checking-in-firefox-for-a-specific-website-domain) of disabling it on the HTML.