4 OE/ORA UI fixes to make it easier to use

@jbau - squashed version of #980, @VikParuchuri gave approval once I addressed his last comment, which I now have...

- require confirmation when clicking Open Ended reset button
- require confirmation when submitting Open Ended essay
- give students a convenient link to peer grading after they've submitted work
- disable text feedback area during calibration and let users know that they should give feedback when grading
- fixed error message typos
- addressing PR comments
- removing unneeded coffeescript variable definition