Add a styleguide page

There are now quite a few frontend components in the app.

It’s good to have a reference for them to:
- document for developers what patterns are available and how they are used
- check for regressions
  - when working on one variation of a pattern you can check that your changes
    don’t break other variations of it
  - when changing the arguments to a pattern you can check that this doesn’t
    change the expected arguments already in use

This commit adds a single page (`/_styleguide`) which has examples of all the
patterns.

tests/app/main/views/test_styleguide.py

@@ -0,0 +1,4 @@
+def test_styleguide_can_render(notifications_admin):
+    response = notifications_admin.test_client().get('/_styleguide')
+
+    assert response.status_code == 200