Group choices for scheduling a job by day

The options for scheduling a job by time should be grouped by day,
because a long list of 96 options is not very usable.

On the server side, this commit generates label for the next 4 days in
a friendly format (ie today/tomorrow/Sunday/Monday)

The Javascript component for choosing a time was built in a kind of
old-school jQuery way, where it manipulated the elements on the page.
The complexity of introducing groups of options was just too much for
this pattern, because it involves storing a lot of state in the DOM.

This commit completely rewrites the JS to:

- read the initial options and groups from the HTML and store them
  in the object
- use Hogan to completely re-render the UI from a series of Mustache
  templates, each of which represents a state of the UI and takes the
  inital options and groups
- filter the choices to show when the today/tomorrow/… buttons are
  clicked

app/main/forms.py

@@ -342,6 +342,7 @@ class ChooseTimeForm(Form):
                 get_furthest_possible_scheduled_time()
             )
         ]
+        self.scheduled_for.categories = get_next_days_until(get_furthest_possible_scheduled_time())
 
     scheduled_for = RadioField(
         'When should Notify send these messages?',