Generate labels for attribute automatically

I'm not sure why it isn't already done by foundation's form builder. It
doesn't make any sense to change an ID of a form field without changing
the `for` attribute of its label.

app/views/tracking/progress_bars/_form.html.erb

@@ -19,8 +19,10 @@
   <% progress_options = { min: ProgressBar::RANGE.min, max: ProgressBar::RANGE.max, step: 1 } %>
   <div class="row">
     <div class="small-12 medium-6 large-2 column">
+      <%= f.label :percentage %>
       <%= f.text_field :percentage, { type: :range,
                                       id: "percentage_range",
+                                      label: false,
                                       class: "column" }.merge(progress_options) %>
     </div>