Show all pages of a letter everywhere

Right now showing all the pages in full is the only way we have of
showing a letter that makes sense to our users. Maybe in the future we
show some kind of truncated version, but the end of the first page is
not a good place to truncate the letter.

This commit just extracts the code for showing multiple pages from the
template view, refactors it for reuse, and includes it in the send
views.

tests/app/test_template_previews.py

@@ -4,7 +4,7 @@ from functools import partial
 from unittest.mock import Mock
 from notifications_utils.template import LetterPreviewTemplate
 
-from app.template_previews import TemplatePreview
+from app.template_previews import TemplatePreview, get_page_count_for_letter
 
 
 @pytest.mark.parametrize('partial_call, expected_page_argument', [
@@ -65,3 +65,32 @@ def test_from_database_object_makes_request(
     headers = {'Authorization': 'Token my-secret-key'}
 
     request_mock.assert_called_once_with(expected_url, json=data, headers=headers)
+
+
+@pytest.mark.parametrize('template_type', [
+    'email', 'sms'
+])
+def test_page_count_returns_none_for_non_letter_templates(template_type):
+    assert get_page_count_for_letter({'template_type': template_type}) is None
+
+
+@pytest.mark.parametrize('partial_call, expected_template_preview_args', [
+    (
+        partial(get_page_count_for_letter),
+        ({'template_type': 'letter'}, None)
+    ),
+    (
+        partial(get_page_count_for_letter, values={'foo': 'bar'}),
+        ({'template_type': 'letter'}, {'foo': 'bar'})
+    ),
+])
+def test_page_count_unpacks_from_json_response(
+    mocker,
+    partial_call,
+    expected_template_preview_args,
+):
+    mock_template_preview = mocker.patch('app.template_previews.TemplatePreview.from_database_object')
+    mock_template_preview.return_value = (b'{"count": 99}', 200, {})
+
+    assert partial_call({'template_type': 'letter'}) == 99
+    mock_template_preview.assert_called_once_with(*expected_template_preview_args, filetype='json')