LP1760940: Turn off browser spell-check on flat text editor

LP1760940: Turn off browser spell-check on flat text editor

This is because there are way too many false positives for the
spell-check to be useful (e.g. codes, subfield delimiters are reported
as spelling errors)

To test:
* Open a record in the Web client in a recent version of Firefox.
* Click MARC Edit.
* Turn on the Flat Text Editor.
* Click inside the editor so that it is in focus.
* Confirm that that no red wavy spelling error underlines appear.

Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>