Adds a feature to the teacher page to pause updating responses

This adds a button to the teacher view of a prompt that allows the
teacher to "freeze" updating the dots.  It also refactors a little bit
of the firebase code in prompt_module in order to make it easier for
this to happen.  Additionally this adds the style "absolute-right" to
the application.scss style sheet that puts an element into absolute
positioning and on the, er, right.