Fix for funky persist bug

The bug that we just recently discovered (making a new assignment without having chosen to persist groups, or even to have students work in groups, resulted in it persisting groups anyway) has been patched in this PR.

It's likely due to it seeing an item being chosen in the `select` dropdown for the persist groups option, despite it not being checked. This has been resolved by having the `select` tag `disabled` unless the option has been checked.

Also moved an AJAX call from the HTML file to the JS file.
:+1: unobtrusive_javascript++, but the ERB tag won't work here because the containing file is a pure .js file not a .js.erb. The Rails way of doing this is by adding a `data-*` attirbute (e.g., data-path='<%= ..._path %>' in the view) to the event-triggering DOM element in the view, let JavaScript get the value of the attribute and leave no trace of Rails-specific code in JavaScript (we should rarely need template files like *.js.erb if we stay unobtrusive).

(BTW the Ruby style guide suggests omitting parenthesis when a method invocation involves no argument.)

A specific Railscast that talks about this, for those who are interested, which also talks about the use of `content_tag` for passing complex JSON data to JavaScript:
http://railscasts.com/episodes/324-passing-data-to-javascript