Fix enroll-multiple button

Fix i18n syntax so that "Enroll multiple students" displays properly.
Currently (on edx.org) it displays as a button with "$_(" on it and doesn't work because the post request uses the button string to trigger an action.

Also, this means that the buttons will NOT work if internalization actually occurs because it will send a translated string up to the server. As a worst case scenario, if there is any language which for example translates "Enroll multiple students" into "Unenroll multiple students" then that would be problematic.

@adampalay @jkarni