Rename "app_" fixture to "notify_admin"

This naming was introduced in 2016 without explanation [1]. I find it
confusing because:

- It's reminiscent of "_app", which is a Python convention indicating
the variable is internal, so maybe avoid using it.

- It suggests there's some other "app" fixture I should be using (there
isn't, though).

The Python style guide describes using an underscore suffix to avoid
clashes with inbuilt names [1], which is sort of applicable if we need
to import the "app" module [2]. However, we can also avoid clashes by
choosing a different name, without the strange underscore.

[1]: 3b1d521c10
[2]: 78824f54fd/tests/app/main/views/test_forgot_password.py (L5)

tests/app/main/test_choose_time_form.py

@@ -5,7 +5,7 @@ from app.main.forms import ChooseTimeForm
 
 
 @freeze_time("2016-01-01 11:09:00.061258")
-def test_form_contains_next_24h(app_):
+def test_form_contains_next_24h(notify_admin):
 
     choices = ChooseTimeForm().scheduled_for.choices
 
@@ -34,12 +34,12 @@ def test_form_contains_next_24h(app_):
 
 
 @freeze_time("2016-01-01 11:09:00.061258")
-def test_form_defaults_to_now(app_):
+def test_form_defaults_to_now(notify_admin):
     assert ChooseTimeForm().scheduled_for.data == ''
 
 
 @freeze_time("2016-01-01 11:09:00.061258")
-def test_form_contains_next_three_days(app_):
+def test_form_contains_next_three_days(notify_admin):
     assert ChooseTimeForm().scheduled_for.categories == [
         'Later today', 'Tomorrow', 'Sunday', 'Monday'
     ]