Let users choose when to end a broadcast

Different emergencies will need broadcasts to last for a variable amount
of time. We give users some control over this by letting them stop a
broadcast early. But we should also let them set a maximum broadcast
time, for:
- when the duration of the danger is known
- when the broadcast has been live long enough to alert everyone who
  needs to know about it

This code re-uses the pattern for scheduling jobs, which has some
constraints that are probably OK for now:
- end time is limited to an hour
- longest duration is 3 whole days (eg if you start broadcasting Friday
  you have the choice of Saturday, Sunday and all of Monday, up to
  midnight)

tests/app/main/test_choose_time_form.py

@@ -11,19 +11,19 @@ def test_form_contains_next_24h(app_):
 
     # Friday
     assert choices[0] == ('', 'Now')
-    assert choices[1] == ('2016-01-01T12:00:00.061258', 'Today at midday')
-    assert choices[13] == ('2016-01-02T00:00:00.061258', 'Today at midnight')
+    assert choices[1] == ('2016-01-01T12:00:00', 'Today at midday')
+    assert choices[13] == ('2016-01-02T00:00:00', 'Today at midnight')
 
     # Saturday
-    assert choices[14] == ('2016-01-02T01:00:00.061258', 'Tomorrow at 1am')
-    assert choices[37] == ('2016-01-03T00:00:00.061258', 'Tomorrow at midnight')
+    assert choices[14] == ('2016-01-02T01:00:00', 'Tomorrow at 1am')
+    assert choices[37] == ('2016-01-03T00:00:00', 'Tomorrow at midnight')
 
     # Sunday
-    assert choices[38] == ('2016-01-03T01:00:00.061258', 'Sunday at 1am')
+    assert choices[38] == ('2016-01-03T01:00:00', 'Sunday at 1am')
 
     # Monday
-    assert choices[84] == ('2016-01-04T23:00:00.061258', 'Monday at 11pm')
-    assert choices[85] == ('2016-01-05T00:00:00.061258', 'Monday at midnight')
+    assert choices[84] == ('2016-01-04T23:00:00', 'Monday at 11pm')
+    assert choices[85] == ('2016-01-05T00:00:00', 'Monday at midnight')
 
     with pytest.raises(IndexError):
         assert choices[