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

tests/app/main/test_choose_time_form.py

@@ -36,3 +36,10 @@ def test_form_contains_next_24h(app_):
 @freeze_time("2016-01-01 11:09:00.061258")
 def test_form_defaults_to_now(app_):
     assert ChooseTimeForm().scheduled_for.data == ''
+
+
+@freeze_time("2016-01-01 11:09:00.061258")
+def test_form_contains_next_three_days(app_):
+    assert ChooseTimeForm().scheduled_for.categories == [
+        'Today', 'Tomorrow', 'Sunday', 'Monday'
+    ]