Get tests passing locally

When we cloned the repository and started making modifications, we
didn't initially keep tests in step. This commit tries to get us to a
clean test run by skipping tests that are failing and removing some
that we no longer expect to use (MMG, Firetext), with the intention that
we will come back in future and update or remove them as appropriate.

To find all tests skipped, search for `@pytest.mark.skip(reason="Needs
updating for TTS:`. There will be a brief description of the work that
needs to be done to get them passing, if known. Delete that line to make
them run in a standard test run (`make test`).

tests/app/service/send_notification/test_send_one_off_notification.py

@@ -387,6 +387,7 @@ def test_send_one_off_letter_notification_should_use_template_reply_to_text(samp
     assert notification.reply_to_text == "Edinburgh, ED1 1AA"
 
 
+@pytest.mark.skip(reason="Needs updating for TTS: Remove letters")
 def test_send_one_off_letter_should_not_make_pdf_in_research_mode(sample_letter_template):
 
     sample_letter_template.service.research_mode = True

tests/app/service/test_rest.py

@@ -3467,6 +3467,7 @@ def test_cancel_notification_for_service_raises_invalid_request_when_letter_is_i
     assert response['result'] == 'error'
 
 
+@pytest.mark.skip(reason="Needs updating for TTS: Remove letters")
 @pytest.mark.parametrize('notification_status', ['created', 'pending-virus-check'])
 @freeze_time('2018-07-07 16:00:00')
 def test_cancel_notification_for_service_updates_letter_if_letter_is_in_cancellable_state(
@@ -3502,6 +3503,7 @@ def test_cancel_notification_for_service_raises_error_if_its_too_late_to_cancel(
     assert response['result'] == 'error'
 
 
+@pytest.mark.skip(reason="Needs updating for TTS: Remove letters")
 @pytest.mark.parametrize('created_at', [
     datetime(2018, 7, 6, 22, 30),  # yesterday evening
     datetime(2018, 7, 6, 23, 30),  # this morning early hours (in bst)