Sets up databases that can be used for development

This changes config/database.yml so that it uses the sqlite3 databases
for development and testing instead of the postgres database that Riley
  had set up.  It keeps the postgres database for production.  There
  doesn't seem to be much reason to insist on using postgres for
  development and testing, though I could be wrong.

  It also modifies the Gemfile by adding a development
  group that loads the gem for sqlite3 and puts the pg gem in a
  production group.
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.  I'm not entirely sure if this is done in the most
efficient manner.

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.