Modify to display ball_holder of Progress's in style

app/assets/stylesheets/style.css
- Add '.finished'.
app/helpers/application_helper.rb
- Add ball_holder_display().
app/views/progresses/_progress.html.erb
app/views/progresses/index.html.erb
app/views/projects/index.html.erb
- Modify to use helper ball_holder_display() to display
  ball_holder of Progress's.