Improvements to the write answer feature

This is mostly to the display:
* Hide the “......” prompt when write answer is off
* Show only the correct answer when nothing is typed, including in the write answer off case
* Show the correct answer normally instead of marked green when write answer is off.
* Add class `typeOff` when write answer is off, to allow hiding the correct answer via CSS.

(My Emacs also removed a few trailing whitespaces.)